Filed Pursuant to Rule 424(b)(5)
Registration Statement Nos. 333-227514 and 333-227514-01

The information in this preliminary prospectus supplement is not complete and may be changed. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell these securities and are not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject to completion, dated April 30, 2019

Preliminary Prospectus Supplement

(To Prospectus dated September 25, 2018)

Aon Corporation

$            % Senior Notes due

With a full and unconditional guarantee as to payment of

principal and interest by Aon plc

Aon Corporation is offering $             aggregate principal amount of         % senior notes due                (the “Notes”). The Notes will mature on             . Aon Corporation will pay interest on the Notes on each             and             , commencing on                     , 2019. The Notes will be issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof. Aon Corporation may redeem all of the Notes at any time, and some of the Notes from time to time, at the redemption prices set forth in this prospectus supplement under “Description of the Securities—Optional Redemption.” Aon Corporation may also redeem all of the Notes at a redemption price equal to 100% of the principal amount of the Notes plus accrued and unpaid interest, if any, to the redemption date in the event of certain changes in respect of withholding taxes applicable to the Guarantee, as described in this prospectus supplement under “Description of the Securities—Optional Tax Redemption.”

The Notes will be fully and unconditionally guaranteed (the “Guarantee” and, together with the Notes, the “Securities”) by Aon Corporation’s indirect parent, Aon plc.

The Notes will be Aon Corporation’s general unsecured and unsubordinated obligations and will rank equally with each other and with all of Aon Corporation’s other existing and future unsecured and unsubordinated obligations. The Notes will not have the benefit of all of the covenants applicable to certain of Aon Corporation’s existing unsecured senior indebtedness. The Notes will be effectively subordinated to any secured indebtedness Aon Corporation may have or incur in the future to the extent of the value of the assets securing any such indebtedness. The Notes will be structurally subordinated to the indebtedness and all other obligations of Aon Corporation’s subsidiaries.

The Guarantee will be a general unsecured and unsubordinated obligation of Aon plc and will rank equally with all of Aon plc’s other existing and future unsecured and unsubordinated obligations. The Guarantee will not have the benefit of all of the covenants applicable to certain of Aon plc’s existing unsecured senior debt. The Guarantee will be effectively subordinated to any secured indebtedness Aon plc may have or incur in the future to the extent of the value of the assets securing any such indebtedness. The Guarantee will be structurally subordinated to the indebtedness and all other obligations of Aon plc’s subsidiaries.

Investing in the Securities involves a high degree of risk. See “Risk Factors” beginning on page S-6 of this prospectus supplement.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these Securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Interest on the Notes will accrue from                     , 2019.

Currently, there is no public market for the Notes, and we currently have no intention to apply to list the Notes on any securities exchange or to seek their admission to trading on any automated quotation system.

The underwriters expect to deliver the Securities for purchase on or about                     , 2019, which is the         business day following the date of this prospectus supplement, in book-entry form through the facilities of The Depository Trust Company and its participants, including Clearstream Banking, S.A. and Euroclear Bank S.A./N.V.

Joint Book-Running Managers

The date of this prospectus supplement is                     , 2019.

CONTENTS

Prospectus Supplement

Prospectus

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Neither we nor the underwriters have authorized anyone to provide any information other than that which is contained or incorporated by reference in this prospectus supplement, the accompanying prospectus or any free writing prospectus prepared by or on behalf of us or to which we have referred you. Neither we nor the underwriters take any responsibility for, or provide any assurance as to, the reliability of any other information that others may give you. No offer to sell these Securities is being made in any jurisdiction where the offer or sale is not permitted. The information contained in this prospectus supplement, the accompanying prospectus, any free writing prospectus or any document incorporated by reference is accurate as of the date of the document in which the information appears. Our business, financial condition, results of operations and prospects may have changed after any of such dates.

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ABOUT THIS PROSPECTUS SUPPLEMENT

This document consists of two parts. The first part is this prospectus supplement, which describes the specific terms of this offering. The second part is the accompanying prospectus, which describes more general information, some of which may not apply to this offering. You should read both this prospectus supplement and the accompanying prospectus, together with the documents incorporated by reference and the additional information described below under the heading “Where You Can Find More Information.”

If the description of this offering varies between this prospectus supplement and the accompanying prospectus, you should rely on the information in this prospectus supplement.

Any statement made in this prospectus supplement or in a document incorporated or deemed to be incorporated by reference in this prospectus supplement will be deemed to be modified or superseded for purposes of this prospectus supplement to the extent that a statement contained in this prospectus supplement or in any other subsequently filed document that is also incorporated or deemed to be incorporated by reference in this prospectus supplement or the accompanying prospectus modifies or supersedes that statement. Except as so modified or superseded, any statement so modified or superseded will not be deemed to constitute a part of this prospectus supplement. See “Incorporation of Certain Documents by Reference” in this prospectus supplement.

In this prospectus supplement, we use the terms “Aon Corporation” or the “Issuer” to refer to Aon Corporation (not including its subsidiaries), and the terms “Aon,” “we,” “us” and “our” and similar terms to refer to Aon plc and its subsidiaries (including Aon Corporation), unless the context otherwise requires. We use the terms “Aon plc” or the “Parent” or the “Guarantor” to refer to Aon plc, Aon Corporation’s indirect parent and the guarantor of the Notes.

IMPORTANT – EEA RETAIL INVESTORS

The Notes are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the European Economic Area (the “EEA”). For these purposes, a retail investor means a person who is one (or more) of: (i) a retail client as defined in point (11) of Article 4(1) of Directive 214/65/EU (as amended, “MiFID II”); or (ii) a customer within the meaning of Directive 2016/97/EU (as amended, the “Insurance Distribution Directive”), where that customer would not qualify as a professional client as defined in point (1) of Article 4(1) of MiFID II; or (iii) not a qualified investor as defined in Directive 2003/71/EC (as amended, the “Prospectus Directive”). Consequently, no key information document required by Regulation (EU) No 1286/2014 (as amended, the “PRIIPs Regulation”) for offering or selling the Notes or otherwise making them available to retail investors in the EEA has been prepared, and therefore offering and selling the Notes or otherwise making them available to any retail investor in the EEA may be unlawful under the PRIIPs Regulation.

WHERE YOU CAN FIND MORE INFORMATION

We are subject to the information reporting requirements of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). In accordance with the Exchange Act, we file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC file number is 001-07933.

The SEC also maintains a web site that contains reports, proxy statements and other information about issuers, including us, who file electronically with the SEC. The address of that site is www.sec.gov.

This prospectus supplement and the accompanying prospectus, which form a part of the registration statement, do not contain all the information that is included in the registration statement. You will find additional information about us in the registration statement. Any statements made in this prospectus supplement, the accompanying prospectus or any documents incorporated by reference in this prospectus supplement or the

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accompanying prospectus concerning the provisions of legal documents are not necessarily complete and you should read the documents that are filed as exhibits to the registration statement or otherwise filed with the SEC for a more complete understanding of the document or matter.

DISCLOSURE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and any documents incorporated by reference into this prospectus supplement or the accompanying prospectus contain certain statements related to future results, or states our intentions, beliefs and expectations or predictions for the future which are forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended. Forward-looking statements relate to expectations or forecasts of future events. They use words such as “anticipate,” “believe,” “estimate,” “expect,” “forecast,” “project,” “intend,” “plan,” “probably,” “potential,” “looking forward” and other similar terms, and future or conditional tense verbs like “could,” “may,” “might,” “should,” “will” and “would.” You can also identify forward-looking statements by the fact that they do not relate strictly to historical or current facts. For example, we may use forward-looking statements when addressing topics such as: market and industry conditions, including competitive and pricing trends; changes in our business strategies and methods of generating revenue; the development and performance of our services and products; changes in the composition or level of our revenues; our cost structure and the outcome of cost-saving or restructuring initiatives; the outcome of contingencies; dividend policy; the expected impact of acquisitions and dispositions; pension obligations; cash flow and liquidity; expected effective tax rate; future actions by regulators; and the impact of changes in accounting rules. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors. Potential factors that could impact results include:

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Any or all of these forward-looking statements may turn out to be inaccurate, and there are no guarantees about our performance. The factors identified above are not exhaustive. We and our subsidiaries

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operate in a dynamic business environment in which new risks may emerge frequently. Accordingly, you should not place undue reliance on forward-looking statements, which speak only as of the dates on which they are made. We are under no obligation (and expressly disclaim any obligation) to update or alter any forward-looking statement that we may make from time to time, whether as a result of new information, future events or otherwise. Further information about factors that could materially affect us, including our results of operations and financial condition, is contained in the “Risk Factors” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” sections in Part I, Item 1A and in Part II, Item 7, respectively, of our Annual Report on Form 10-K for the year ended December 31, 2018, as filed with the SEC, as such factors may be updated in our periodic filings with the SEC.

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RISK FACTORS

You should carefully consider the risks described below, the risks set forth in the accompanying prospectus and the other information set forth in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein before making an investment decision. These risks include those set forth in the “Risk Factors” section of our Annual Report on Form 10-K for the year ended December 31, 2018 as filed with the SEC, as such factors may be updated in our periodic filings with the SEC, and include risks that could have a material adverse effect on our financial condition, results of operations or cash flows and which could, in turn, impact our ability to perform our respective obligations under the Securities.

Additional risks and uncertainties not presently known to us, or that we currently deem immaterial, may also impair our business operations. The events discussed in the risk factors below, or the risk factors in the accompanying prospectus or the documents incorporated by reference herein or therein, may occur. If they do, our business, results of operations or financial condition could be materially adversely affected. In such an instance, the trading prices of our securities, including the Securities, could decline and you might lose all or part of your investment.

Risks Related to the Notes and the Guarantee

The Notes will be effectively subordinated to all of Aon Corporation’s existing and future secured indebtedness (to the extent of the value of the assets securing any such indebtedness) and to the existing and future indebtedness of Aon Corporation’s subsidiaries, and the Guarantee will be effectively subordinated to all of Aon plc’s existing and future secured indebtedness (to the extent of the value of the assets securing any such indebtedness) and to the existing and future indebtedness of Aon plc’s subsidiaries.

The Notes are not secured by any of Aon Corporation’s assets or the assets of its subsidiaries, and the Guarantee is not secured by any of the assets of Aon plc or the assets of Aon plc’s subsidiaries. As a result, the indebtedness represented by the Notes will effectively be subordinated to any existing and future secured indebtedness of Aon Corporation or its subsidiaries and the indebtedness represented by the Guarantee will effectively be subordinated to any existing and future secured indebtedness of Aon plc or its subsidiaries, in each case to the extent of the value of the assets securing any such indebtedness. As of March 31, 2019, neither Aon Corporation nor Aon plc had any secured indebtedness for borrowed money. As of March 31, 2019, Aon plc’s subsidiaries (other than Aon Corporation) had approximately $4.4 billion of outstanding indebtedness and other liabilities, including trade payables, pensions and other post-employment liabilities, non-current operating lease liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. As of March 31, 2019, Aon Corporation’s subsidiaries had approximately $4.2 billion of outstanding indebtedness and other liabilities, including trade payables, pensions and other post-employment liabilities, non-current operating lease liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. In the event of any distribution or payment of Aon Corporation’s assets or those of Aon plc in any foreclosure, dissolution, winding up, liquidation or reorganization, or other bankruptcy proceeding, any secured creditors of Aon Corporation or of Aon plc, respectively, would have a superior claim to holders of the Notes to the extent of the value of their collateral. In the event of the dissolution, a winding up, liquidation or reorganization, or other bankruptcy proceeding of a subsidiary of Aon Corporation, creditors of that subsidiary would generally have the right to be paid in full before any distribution is made to Aon Corporation or you in respect of the Notes. In the event of a dissolution, winding up, liquidation or reorganization, or other bankruptcy proceeding of a subsidiary of Aon plc, creditors of that subsidiary would generally have the right to be paid in full before any distribution is made to Aon plc or to you in respect of the Guarantee. If any of the foregoing occur, we cannot assure you that there will be sufficient assets to pay amounts due on the Securities.

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We need to maintain adequate liquidity in order to have sufficient cash to meet operating cash flow requirements, repay maturing indebtedness and satisfy other obligations. If we fail to comply with the covenants contained in our various borrowing agreements, our (including Aon Corporation’s) liquidity, results of operations and financial condition may be adversely affected.

Our liquidity is a function of our ability to successfully generate cash flows from operations and improvement therein, access to capital markets and borrowings under our credit agreements. We believe our liquidity (including operating and other cash flows that we expect to generate) will be sufficient to meet operating requirements as they occur; however, our ability to maintain sufficient liquidity going forward depends on our ability to generate cash from operations and access to the capital markets and borrowings, all of which are subject to general economic, financial, competitive, legislative, regulatory and other market factors that are beyond our control.

As of March 31, 2019, Aon plc had two primary committed credit facilities outstanding: its $900 million multi-currency U.S. credit facility expiring in February 2022 (the “February 2022 Facility”) and its $400 million multi-currency U.S. credit facility expiring in October 2022 (the “October 2022 Facility”). These credit facilities are intended to support our commercial paper obligations and our general working capital needs. At March 31, 2019, we had no borrowings outstanding under either of these facilities. At March 31, 2019, we were compliant with the financial covenants contained in our credit facilities. However, failure to comply with material provisions of our covenants in the credit facilities could result in a default under the credit agreements, rendering them unavailable to us and causing a material adverse effect on our (including Aon Corporation’s) liquidity, results of operations and financial condition.

Certain of our financing agreements, including our credit facilities, contain various covenants that limit the discretion of our management in operating our business and could prevent us from engaging in certain potentially beneficial activities, and the violation of these covenants could result in an event of default. The Securities will not have the benefit of all of these covenants.

The restrictive covenants in our financing agreements may impact how we operate our business and prevent us from engaging in certain potentially beneficial activities. For both the February 2022 Facility and the October 2022 Facility, the two most significant covenants require us to maintain (i) a ratio of consolidated EBITDA (earnings before interest, taxes, depreciation and amortization), adjusted for losses that are an unusual nature or infrequently occurring, certain non-cash charges and expenses, restructuring costs, and certain non-recurring acquisition-related fees, charges and expenses to consolidated interest expense and (ii) a ratio of consolidated debt to Adjusted EBITDA. For both facilities, the ratio of Adjusted EBITDA to consolidated interest expense must be at least 4.00 to 1.00. For the both facilities, the ratio of consolidated debt to Adjusted EBITDA must not exceed 3.25 to 1.00, subject to certain exceptions. The indenture governing the Securities does not include any similar covenants. Failure to comply with the covenants contained in the February 2022 Facility, the October 2022 Facility or any of our other existing indebtedness could result in an event of default under either such facility or our other existing indebtedness that, if not cured or waived, could have a material adverse effect on our business, financial condition and results of operations. In the event of certain defaults under the February 2022 Facility, the October 2022 Facility or any of our other indebtedness, the lenders thereunder would not be required to lend any additional amounts to us and could elect to declare all borrowings outstanding, together with accrued and unpaid interest and fees, to be due and payable. If the indebtedness under February 2022 Facility, the October 2022 Facility or any of our other indebtedness, including the Securities, were to be accelerated, there can be no assurance that our assets would be sufficient to repay such indebtedness in full. See “Description of the Securities.”

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The indenture governing the Securities contains no covenants limiting our or the Issuer’s ability to incur future indebtedness, pay dividends or transfer assets among our subsidiaries, and only limited restrictions on our or the Issuer’s ability to engage in other activities, which could adversely affect our or the Issuer’s ability to pay our respective obligations under the Securities.

The indenture does not contain any financial covenants. The indenture will permit us and our subsidiaries (including the Issuer) to incur additional indebtedness, including secured indebtedness. Because the Securities will be unsecured, in the event of any dissolution, winding up, liquidation or reorganization or other bankruptcy proceeding regarding us or the Issuer, whether voluntary or involuntary, the holders of our or the Issuer’s secured indebtedness will be entitled to receive payment to the extent of the value of the assets securing that indebtedness before we or the Issuer can make any payment with respect to the Securities. If any of the foregoing events occurs, we cannot assure you that we or the Issuer will have sufficient assets to pay amounts due on the Securities. As a result, you may receive a payment on the Securities that is less than that which you are entitled to receive or recover nothing if any liquidation, dissolution, reorganization, bankruptcy or other similar proceeding occurs.

The indenture governing the Securities does not limit our, the Issuer’s or our respective subsidiaries’ ability to issue or repurchase securities, pay dividends, incur intercompany liabilities or engage in transactions with affiliates. Our and the Issuer’s ability to use funds for numerous purposes may limit the funds available to pay our or the Issuer’s obligations under the Securities.

The Securities lack a developed public market, and we do not intend to list the Securities on any securities exchange or quotation system.

The Notes are new securities for which there currently is no established market. We do not intend to apply for the Notes to be listed on any securities exchange or to arrange for the Notes to be quoted on any quotation system. There can be no assurance regarding the future development of a market for the Securities or the ability of holders of the Securities to sell their Securities or the price at which such holders may be able to sell their Securities. If such a market were to develop, the Securities could trade at prices that may be higher or lower than the initial offering price depending on many factors, including, among other things, prevailing interest rates, our or the Issuer’s operating results or financial condition and the market for similar securities. Underwriters, broker-dealers and agents that participate in the distribution of the Securities may make a market in the Securities as permitted by applicable laws and regulations but will have no obligation to do so, and any such market-making activities with respect to the Securities may be discontinued at any time without notice. Therefore, there can be no assurance as to the liquidity of any trading market for the Securities or that an active public market for the Securities will develop. See “Underwriting.”

Our and the Issuer’s credit ratings may not reflect all risks of an investment in the Securities, and are subject to change.

The credit ratings of the Securities may not reflect the potential impact of all risks related to structure and other factors on any trading market for, or trading value of, the Securities. In addition, real or anticipated changes in our or the Issuer’s credit ratings, which could result from any number of factors (including the modification by a credit rating agency of the criteria or methodology it applies to particular issuers, including us or the Issuer), will generally affect any trading market for, or trading value of, the Securities.

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USE OF PROCEEDS

The net proceeds to us from this offering after deducting the underwriting discounts and estimated offering expenses payable by us are expected to be approximately $            million. We intend to use the net proceeds from this offering to pay down a portion of outstanding commercial paper and for general corporate purposes.

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CAPITALIZATION OF AON PLC

The following table sets forth Aon plc’s capitalization as of March 31, 2019, on an actual basis and on an as adjusted basis to give effect to this offering as if it had occurred on such date. You should read the data set forth in the table below in conjunction with “Summary—Selected Historical Financial Data” and “Use of Proceeds” appearing elsewhere in this prospectus supplement, as well as “Management’s Discussion and Analysis of Financial Condition and Results of Operations,” which is incorporated by reference into this prospectus supplement from our Annual Report on Form 10-K for the year ended December 31, 2018.

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DESCRIPTION OF THE SECURITIES

The following description of the particular terms of the Securities offered by this prospectus supplement supplements, and to the extent inconsistent therewith, replaces the description of the general terms and provisions of the Securities set forth under the caption “Description of Debt Securities and Guarantees” in the accompanying prospectus. Terms used in this prospectus supplement that are otherwise not defined have the meanings given to them in the accompanying prospectus.

Aon Corporation will issue $             aggregate principal amount of         % senior notes due                (the “Notes”) pursuant to the indenture dated as of December 3, 2018 among Aon Corporation, as issuer, Aon plc, as guarantor, and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Trustee”), as supplemented by an officers’ certificate, dated as of                     , 2019. We refer to the original indenture, as supplemented, as the indenture. The following is a summary of the material provisions of the indenture governing the Securities. It does not include all of the provisions of the indenture. We urge you to read the indenture because it, not this description, defines your rights. The terms of the Securities include those stated in the indenture and those made part of the indenture by reference to the Trust Indenture Act of 1939, as amended (the “TIA”). A copy of the indenture may be obtained from Aon Corporation or the Trustee.

Aon Corporation will issue the Notes in fully registered form in denominations of $2,000 and integral multiples of $1,000 in excess thereof. The Trustee will initially act as paying agent and registrar for the Notes. The Notes may be presented for registration of transfer and exchange at the offices of the registrar. Aon Corporation may change the paying agent and registrar without notice to holders of the Notes. It is expected that Aon Corporation will pay principal and interest (and premium, if any) on the Notes (and, as necessary, Aon plc will pay such amounts in relation to the Guarantee) at the Trustee’s corporate office by wire transfer, if book-entry at DTC, or check mailed to the registered address of holders.

Principal, Maturity and Interest

The Notes will mature on             . $             in aggregate principal amount of Notes will be issued in this offering. After the issue date of the Notes, additional notes having the same terms and conditions as the Notes in all respects (other than the issue date, public offering price, and to the extent applicable, first date of interest accrual and first interest payment date of such notes) (“Additional Notes”) may be issued from time to time; provided, however, that if the Additional Notes are not fungible with the Notes for U.S. federal income tax purposes, the Additional Notes will have a separate CUSIP number. The Notes and any Additional Notes that are actually issued will be treated as a single class for all purposes under the indenture, including, without limitation, as to waivers, amendments, redemptions and any applicable offers to purchase. Unless the context otherwise requires, for all purposes of the indenture and this “Description of the Securities,” references to the Notes include any Additional Notes actually issued.

Interest on the Notes will accrue at the rate of     % per annum and will be payable semi-annually in arrears in cash on each                  and                 , commencing on                     , 2019, to the persons who are registered holders at the close of business on                 or                 , as the case may be, immediately preceding the applicable interest payment date. Interest on the Notes will accrue from the most recent date to which interest has been paid or, if no interest has been paid, from and including the date of issuance to but excluding the actual interest payment date.

Interest will be computed on the basis of a 360-day year composed of twelve 30-day months.

Optional Redemption

Aon Corporation may at its option redeem all of the Notes at any time and some of the Notes from time to time, at a redemption price equal to the greater of:

(i)        100% of the principal amount of the Notes being redeemed; and

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(ii)        the sum of the present values of the remaining scheduled payments of principal and interest thereon (not including any portion of such payments of interest accrued as of the redemption date), discounted to the date of redemption on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the Treasury Rate (as defined below), plus                basis points (     %),

plus, in either case, accrued and unpaid interest on the principal amount of the Notes being redeemed to but excluding the redemption date.

On or after                (                months prior to maturity), Aon Corporation may redeem any or all of the Notes at a redemption price equal to 100% of the principal amount of the Notes being redeemed, plus accrued and unpaid interest on the principal amount of the Notes being redeemed to, but excluding, the redemption date.

Notwithstanding the foregoing, installments of interest on Notes being redeemed that are due and payable on interest payment dates falling on or prior to a redemption date will be payable on the interest payment date to the registered holders as of the close of business on the relevant record date according to the Notes and the indenture.

“Comparable Treasury Issue” means the United States Treasury security selected by the Quotation Agent as having a maturity comparable to the remaining term of the Notes to be redeemed that would be utilized, at the time of selection and in accordance with customary financial practice, in pricing new issues of corporate debt securities of comparable maturity to the remaining term of such Notes.

“Comparable Treasury Price” means, with respect to any redemption date, (i) the average of three Reference Treasury Dealer Quotations for such redemption date, after excluding the highest and lowest such Reference Treasury Dealer Quotations, or (ii) if the Quotation Agent is given fewer than five such Reference Treasury Dealer Quotations, the average of all such quotations, or (iii) if only one Reference Treasury Dealer Quotation is received, such quotation.

“Quotation Agent” means the Reference Treasury Dealer appointed by us.

“Reference Treasury Dealer” means each of Citigroup Global Markets Inc., Barclays Capital Inc. and Credit Suisse Securities (USA) LLC (or their respective affiliates that are primary U.S. government securities dealers in New York City, each of which we refer to as a Primary Treasury Dealer) and their respective successors and any other nationally recognized investment banking firm that is a Primary Treasury Dealer appointed from time to time by us; provided, however, that if any of the foregoing shall cease to be a Primary Treasury Dealer, we will substitute therefor another Primary Treasury Dealer.

“Reference Treasury Dealer Quotations” means, with respect to each Reference Treasury Dealer and any redemption date, the average, as determined by the Quotation Agent, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to the Quotation Agent by such Reference Treasury Dealer at 5:00 p.m., New York City time, on the third business day preceding such redemption date.

“Treasury Rate” means, with respect to any redemption date, the rate per annum equal to the semi-annual equivalent yield to maturity of the Comparable Treasury Issue, assuming a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for such redemption date.

Notice of any redemption described under “—Optional Redemption” will be sent at least 30 days but not more than 90 days before the redemption date to each holder of the Notes to be redeemed. Unless the Issuer and Aon plc default in payment of the redemption price, on and after the redemption date, interest will cease to accrue on the Notes or portions thereof called for redemption. If less than all the Notes are to be redeemed, the

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Notes to be redeemed shall be selected by lot if the Notes are in definitive form, and, if the Notes are in global form then in accordance with the procedures of the Depository Trust Company.

Guarantee

Under the Guarantee, Aon plc will fully and unconditionally guarantee the due and punctual payment of the principal, interest, premium (if any) and all other amounts due under the Indenture and on the Notes when the Notes become due and payable, whether at maturity, pursuant to optional redemption, by acceleration or otherwise, in each case after any applicable grace periods or notice requirements, according to the terms of the Notes.

The obligations of Aon plc under the Guarantee will be unconditional, regardless of the enforceability of the Notes, and will not be discharged until all obligations under the Notes and the indenture are satisfied. Holders of the Notes may proceed directly against Aon plc under the Guarantee if an event of default affecting the Notes occurs without first proceeding against Aon Corporation.

Ranking

The Notes will be:

The Guarantee will be:

As noted above, the Notes will be effectively subordinated to all of Aon Corporation’s existing and future secured indebtedness to the extent of the value of the assets securing any such indebtedness and structurally subordinated to all of Aon Corporation’s subsidiaries’ existing and future obligations. In addition, the

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Guarantee will be effectively subordinated to all of Aon plc’s existing and future secured indebtedness to the extent of the value of the assets securing any such indebtedness and structurally subordinated to all of Aon plc’s subsidiaries’ existing and future obligations. See “Risk Factors—The Notes will be effectively subordinated to all of Aon Corporation’s existing and future secured indebtedness (to the extent of the value of the assets securing any such indebtedness) and to the existing and future indebtedness of Aon Corporation’s subsidiaries, and the Guarantee will be effectively subordinated to all of Aon plc’s existing and future secured indebtedness (to the extent of the value of the assets securing any such indebtedness) and to the existing and future indebtedness of Aon plc’s subsidiaries.” As of March 31, 2019, Aon Corporation had no secured indebtedness for borrowed money and had approximately $7,822 million of consolidated outstanding indebtedness and other liabilities, including trade payables, pension and other post-employment liabilities, non-current operating lease liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. As of March 31, 2019, Aon Corporation’s subsidiaries had approximately $4,220 million of outstanding indebtedness and other liabilities, including trade payables, pension and other post-employment liabilities, non-current operating lease liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. These liabilities constitute approximately 53.9% of Aon Corporation’s total consolidated liabilities. As of March 31, 2019, Aon plc had no secured indebtedness for borrowed money and had approximately $12,861 million of consolidated outstanding indebtedness and other liabilities, including trade payables, pension and other post-employment liabilities, non-current operating lease liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. As of March 31, 2019, Aon plc’s subsidiaries had no secured indebtedness for borrowed money and had approximately $8,030 million of outstanding indebtedness and other liabilities including trade payables, pension and other post-employment liabilities, non-current operating lease liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities, constituting approximately 62.4% of Aon plc’s total consolidated liabilities.

Payment of Additional Amounts

Payments made by Aon plc in respect of the Guarantee will be made free and clear of and without withholding or deduction for or on account of any present or future income, stamp or other tax, duty, levy, impost, assessment or other governmental charge of any nature whatsoever imposed or levied by or on behalf of the government of the United Kingdom, of any territory of the United Kingdom or by any authority or agency therein or thereof having the power to tax, which we refer to collectively as ‘‘Taxes,’’ unless Aon plc is required to withhold or deduct Taxes by law.

If Aon plc is required to withhold or deduct any amount for or on account of Taxes from any payment made with respect to the Guarantee, Aon plc will pay such additional amounts as may be necessary so that the net amount received by each beneficial owner (including additional amounts) after such withholding or deduction will not be less than the amount the beneficial owner would have received if the Taxes had not been withheld or deducted; provided that no additional amounts will be payable with respect to Taxes:

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nor shall additional amounts be paid with respect to any payment in respect of the Guarantee to any such holder who is a fiduciary or a partnership or who is other than the sole beneficial owner of such payment to the extent a beneficiary or settlor with respect to such fiduciary or a member of such partnership or a beneficial owner would not have been entitled to such additional amounts had it been the holder of the Note.

All references in this prospectus supplement and the accompanying prospectus, other than under ‘‘Description of Debt Securities and Guarantees—Defeasance’’ in the accompanying prospectus, to the payment of the principal of or premium, if any, or interest, if any, on or the net proceeds received on the sale or exchange of, any Notes or any payment made under the Guarantee shall be deemed to include additional amounts to the extent that, in that context, additional amounts are, were or would be payable.

Aon plc’s obligations to pay additional amounts if and when due will survive the termination of the indenture and the payment of all other amounts in respect of the Notes.

Optional Tax Redemption

Aon Corporation may redeem the Notes in whole, but not in part, at its option at any time prior to maturity, upon the giving of not less than 30 nor more than 90 days’ notice of tax redemption to the holders, at a redemption price equal to the principal amount of the Notes plus accrued and unpaid interest, if any, to the redemption date, if:

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No notice of any such redemption may be given earlier than 90 days prior to the earliest date on which Aon plc would be obligated to pay any additional amounts.

Prior to the delivery of any notice of redemption, Aon Corporation will deliver to the Trustee an officer’s certificate stating that it is entitled to effect or cause a redemption and setting forth a statement of facts showing that the conditions precedent of the right so to redeem or cause such redemption have occurred, and in the case of a redemption based on an opinion of independent counsel referred to in the second bullet above, such independent counsel’s opinion. Delivery of any notice of redemption will be conclusive and binding on the holders of the Securities being redeemed.

Any notice of redemption will be irrevocable once an officer’s certificate has been delivered to the Trustee.

Governing Law

The indenture and the Securities will be governed by, and construed in accordance with, the laws of the State of New York.

Concerning Our Relationship with the Trustee

Aon plc and Aon Corporation have commercial deposits and custodial arrangements with The Bank of New York Mellon Trust Company, N.A., or “BNYM,” and may have borrowed money from BNYM or its affiliates in the normal course of business. Aon plc and Aon Corporation may enter into similar or other banking relationships with BNYM or its affiliates in the future in the normal course of business. In addition, Aon plc and Aon Corporation have provided brokerage and other insurance services in the ordinary course of our business for BNYM or its affiliates. BNYM may also act as trustee with respect to other debt securities issued by Aon plc and Aon Corporation.

Offers to Purchase; Open Market Purchases

Neither Aon plc nor Aon Corporation is required to make any sinking fund payments or any offers to purchase with respect to the Notes or the Guarantee. Aon plc or Aon Corporation may at any time and from time to time purchase Notes in the open market or otherwise.

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MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES

The following discussion is a summary of the material U.S. federal income tax consequences of the purchase, ownership and disposition of the Notes issued pursuant to this offering, but does not purport to be a complete analysis of all potential tax effects. The effects of other U.S. federal tax laws, such as estate and gift tax laws, and any applicable state, local or foreign tax laws are not discussed. This discussion is based on the Code, Treasury Regulations promulgated thereunder, judicial decisions, and published rulings and administrative pronouncements of the U.S. Internal Revenue Service (the “IRS”), in each case in effect as of the date hereof. These authorities may change or be subject to differing interpretations. Any such change or differing interpretation may be applied retroactively in a manner that could adversely affect a holder of the Notes. We have not sought and will not seek any rulings from the IRS regarding the matters discussed below. There can be no assurance the IRS or a court will not take a contrary position to that discussed below regarding the tax consequences of the purchase, ownership and disposition of the Notes.

This discussion is limited to holders who hold the Notes as “capital assets” within the meaning of Section 1221 of the Code (generally, property held for investment). In addition, this discussion is limited to persons purchasing the Notes for cash at original issue and at their original “issue price” within the meaning of Section 1273 of the Code (i.e., the first price at which a substantial amount of the Notes is sold to the public for cash). This discussion assumes that the Notes are issued at par or with a statutorily defined de minimis amount of original issue discount. This discussion does not address all U.S. federal income tax consequences relevant to a holder’s particular circumstances, including the impact of the Medicare contribution tax on net investment income. In addition, it does not address consequences relevant to holders subject to special rules, including, without limitation:

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If an entity treated as a partnership for U.S. federal income tax purposes holds the Notes, the tax treatment of a partner in the partnership will depend on the status of the partner, the activities of the partnership and certain

determinations made at the partner level. Accordingly, partnerships holding or considering an investment in the Notes and the partners in such partnerships should consult their tax advisors regarding the U.S. federal income tax consequences to them.

INVESTORS SHOULD CONSULT THEIR TAX ADVISORS WITH RESPECT TO THE APPLICATION OF THE U.S. FEDERAL INCOME TAX LAWS TO THEIR PARTICULAR SITUATIONS AS WELL AS ANY TAX CONSEQUENCES OF THE PURCHASE, OWNERSHIP AND DISPOSITION OF THE NOTES ARISING UNDER OTHER U.S. FEDERAL TAX LAWS (INCLUDING ESTATE AND GIFT TAX LAWS), UNDER THE LAWS OF ANY STATE, LOCAL OR NON-U.S. TAXING JURISDICTION OR UNDER ANY APPLICABLE TAX TREATY.

Tax Consequences Applicable to U.S. Holders

Definition of a U.S. Holder

For purposes of this discussion, a “U.S. Holder” is a beneficial owner of a Note that, for U.S. federal income tax purposes, is or is treated as:

Payments of Interest

Interest on a Note generally will be taxable to a U.S. Holder as ordinary income at the time such interest is received or accrued, in accordance with such U.S. Holder’s method of tax accounting for U.S. federal income tax purposes.

Sale or Other Taxable Disposition

A U.S. Holder will recognize gain or loss on the sale, exchange, redemption, retirement or other taxable disposition of a Note generally equal to the difference, if any, between the amount received for the Note in cash or other property valued at fair market value (less amounts attributable to any accrued but unpaid interest, which will be taxable as ordinary income to the extent not previously included in income) and the U.S. Holder’s adjusted tax basis in the Note. A U.S. Holder’s adjusted tax basis in a Note generally will be equal to the amount the U.S. Holder paid for the Note. Any gain or loss will be capital gain or loss, and will be long-term capital gain or loss if the U.S. Holder has held the Note for more than one year at the time of sale or other taxable disposition. Otherwise, such gain or loss will be short-term capital gain or loss. Long-term capital gains recognized by certain non-corporate U.S. Holders, including individuals, generally will be taxable at a reduced rate. The deductibility of capital losses is subject to limitations.

Information Reporting and Backup Withholding

A U.S. Holder generally will be subject to information reporting and backup withholding when the U.S. Holder receives payments on a Note or receives proceeds from the sale or other taxable disposition of a Note (including

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a redemption or retirement of a Note). Certain U.S. Holders are exempt from backup withholding, including corporations and certain tax-exempt organizations. A U.S. Holder will be subject to backup withholding with respect to such payments and proceeds if the U.S. Holder is not otherwise exempt and:

Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules may be allowed as a refund or a credit against a U.S. Holder’s U.S. federal income tax liability, provided the required information is timely furnished to the IRS. U.S. Holders should consult their tax advisors regarding their qualification for an exemption from backup withholding and the procedures for obtaining such an exemption.

Tax Consequences Applicable to Non-U.S. Holders

Definition of a Non-U.S. Holder

For purposes of this discussion, a “Non-U.S. Holder” is a beneficial owner of a Note that is neither a U.S. Holder nor an entity treated as a partnership for U.S. federal income tax purposes.

Payments of Interest

Interest paid on a Note to a Non-U.S. Holder that is not effectively connected with the Non-U.S. Holder’s conduct of a trade or business within the United States generally will not be subject to U.S. federal income tax, or withholding tax of 30% (or such lower rate specified by an applicable income tax treaty), provided that:

If a Non-U.S. Holder does not satisfy the requirements above, such Non-U.S. Holder may be entitled to a reduction in or an exemption from withholding on such interest as a result of any applicable income tax treaty.

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To claim such entitlement, the Non-U.S. Holder must provide the applicable withholding agent with a properly executed IRS Form W-8BEN or W-8BEN-E (or other applicable documentation) claiming a reduction in or exemption from withholding tax under the benefit of an applicable income tax treaty between the United States and the country in which the Non-U.S. Holder resides or is established.

If interest paid to a Non-U.S. Holder is effectively connected with the Non-U.S. Holder’s conduct of a trade or business within the United States (and, if required by an applicable income tax treaty, the Non-U.S. Holder maintains a permanent establishment in the United States to which such interest is attributable), the Non-U.S. Holder will be exempt from the U.S. federal withholding tax described above. To claim the exemption, the Non-U.S. Holder must furnish to the applicable withholding agent a valid IRS Form W-8ECI, certifying that the interest paid on a Note is not subject to withholding tax because it is effectively connected with the conduct of the Non-U.S. Holder of a trade or business within the United States.

Any such effectively connected interest generally will be subject to U.S. federal income tax at the regular graduated U.S. federal income tax rates as if the Non-U.S. Holder were a U.S. Holder, unless an applicable income tax treaty provides otherwise. In addition, a Non-U.S. Holder that is a corporation also may be subject to a branch profits tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on its effectively connected earnings and profits for the taxable year that is attributable to such interest, as adjusted for certain items.

The certifications described above must be provided to the applicable withholding agent prior to the payment of interest and must be updated periodically. Non-U.S. Holders that do not timely provide the applicable withholding agent with the required certification, but that qualify for a reduced rate under an applicable income tax treaty, may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS. Non-U.S. Holders should consult their tax advisors regarding their entitlement to benefits under any applicable income tax treaty.

Sale or Other Taxable Disposition

A Non-U.S. Holder will not be subject to U.S. federal income tax on any gain realized upon the sale, exchange, redemption, retirement or other taxable disposition of a Note (other than amounts allocable to accrued and unpaid interest, which generally will be treated as interest and may be subject to the rules discussed above in “—Payments of Interest”) unless:

Gain of a Non-U.S. Holder described in the first bullet point above generally will be subject to U.S. federal income tax on a net income basis at the regular graduated rates. A Non-U.S. Holder that is a foreign corporation also may be subject to a branch profits tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on its effectively connected earnings and profits for the taxable year that is attributable to such gain, as adjusted for certain items.

Gain of a Non-U.S. Holder described in the second bullet point above will be subject to U.S. federal income tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty), which may be offset by U.S. source capital losses of the Non-U.S. Holder (even though the individual is not considered a resident of the United States), provided the Non-U.S. Holder has timely filed U.S. federal income tax returns with respect to such losses.

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Non-U.S. Holders should consult their tax advisors regarding any applicable income tax treaties that may provide for different rules.

Information Reporting and Backup Withholding

Payments of interest generally will not be subject to backup withholding, provided the applicable withholding agent does not have actual knowledge or reason to know the holder is a United States person and the holder certifies its non-U.S. status as described above under “—Payments of Interest.” However, information returns are required to be filed with the IRS in connection with any interest paid to the Non-U.S. Holder, regardless of whether any tax was actually withheld. In addition, proceeds of the sale or other taxable disposition of a Note (including a retirement or redemption of the Note) within the United States or conducted through certain U.S.-related brokers generally will not be subject to backup withholding or information reporting, if the applicable withholding agent receives the statement described above and does not have actual knowledge or reason to know that such holder is a United States person or the holder otherwise establishes an exemption. Proceeds of a disposition of a Note paid outside the United States and conducted through a non-U.S. office of a non-U.S. broker generally will not be subject to backup withholding or information reporting.

Copies of information returns that are filed with the IRS may also be made available under the provisions of an applicable treaty or agreement to the tax authorities of the country in which the Non-U.S. Holder resides or is established.

Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules may be allowed as a refund or a credit against a Non-U.S. Holder’s U.S. federal income tax liability, provided the required information is timely furnished to the IRS.

Additional Withholding Tax on Payments Made to Foreign Accounts

Withholding taxes may be imposed under Sections 1471 to 1474 of the Code (such Sections commonly referred to as the Foreign Account Tax Compliance Act, or “FATCA”) on certain types of payments made to non-U.S. financial institutions and certain other non-U.S. entities. Specifically, a 30% withholding tax may be imposed on payments of interest on, or (subject to the proposed Treasury Regulations discussed below) gross proceeds from the sale or other disposition of, a Note paid to a “foreign financial institution” or a “non-financial foreign entity” (each as defined in the Code), unless (1) the foreign financial institution undertakes certain diligence and reporting obligations, (2) the non-financial foreign entity either certifies it does not have any “substantial United States owners” (as defined in the Code) or furnishes identifying information regarding each substantial United States owner, or (3) the foreign financial institution or non-financial foreign entity otherwise qualifies for an exemption from these rules. If the payee is a foreign financial institution and is subject to the diligence and reporting requirements in (1) above, it must enter into an agreement with the U.S. Department of the Treasury requiring, among other things, that it undertake to identify accounts held by certain “specified United States persons” or “United States owned foreign entities” (each as defined in the Code), annually report certain information about such accounts, and withhold 30% on certain payments to non-compliant foreign financial institutions and certain other account holders. Foreign financial institutions located in jurisdictions that have an intergovernmental agreement with the United States governing FATCA may be subject to different rules.

Under the applicable Treasury Regulations and administrative guidance, withholding under FATCA generally applies to payments of interest on a Note. While withholding under FATCA would have applied also to payments of gross proceeds from the sale or other disposition of a Note, recently proposed Treasury Regulations eliminate FATCA withholding on payments of gross proceeds entirely. The preamble to these proposed Treasury Regulations specifies that taxpayers generally may rely on these proposed Treasury Regulations until final Treasury Regulations are issued.

Prospective investors should consult their tax advisors regarding the potential application of withholding under FATCA to their investment in the Notes.

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CERTAIN UNITED KINGDOM TAX CONSEQUENCES

The following is a general summary of the U.K. withholding tax consequences in relation to payments of interest on the Notes and payments in respect of the Guarantee and of the U.K. stamp duty and stamp duty reserve tax consequences of the issue or transfer of the Notes. This summary does not deal with other U.K. tax aspects of acquiring, holding or disposing of the Securities. This summary is based upon U.K. tax law and the published practice of HM Revenue and Customs in effect on the date of this prospectus supplement and is subject to any change in law or practice which may take effect after that date (including with retrospective effect).

Holders (or prospective holders) of Securities who are in any doubt as to their tax position should consult their professional advisors.

U.K. Withholding Tax

Payments of interest on the Notes

Interest payments on the Notes made by the Issuer may be made without withholding on account of U.K. income tax provided that they do not have a “U.K. source”. Interest payments on the Notes made by the Issuer would not generally be considered to have a U.K. source where: (i) the Issuer is (under U.K. law) a non-U.K. tax resident, non-U.K. incorporated company; (ii) the interest is not expressed to be the interest of a U.K. branch; (iii) the funds for the payments do not come from the U.K.; and (iv) no debt is secured on U.K. property. Interest payments on the Notes made by the Issuer should not have a U.K. source solely due to the Guarantor in respect of the Notes being resident in the U.K. The Issuer does not currently intend to withhold for or on account of United Kingdom income tax from payments on interest on the Notes.

In all other circumstances, payments of interest on the Notes may be subject to withholding on account of U.K. income tax at the basic rate (currently 20 per cent), subject to such relief as may be available under the provisions of any applicable double tax treaty or any other relief or exemption that may apply.

Payments in respect of the Guarantee

Depending on the correct analysis under U.K. tax law of payments in respect of the Guarantee, it is possible that such payments would be subject to withholding on account of U.K. income tax at the basic rate (currently 20 per cent), subject to such relief as may be available under the provisions of any applicable double tax treaty or any other exemption which may apply.

Stamp duty and stamp duty reserve tax

No U.K. stamp duty or stamp duty reserve tax should be payable on the issue or transfer of the Notes.

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BOOK-ENTRY, DELIVERY AND FORM

We have obtained the information in this section concerning The Depository Trust Company (“DTC”), Clearstream Banking, S.A. (“Clearstream, Luxembourg”) and Euroclear Bank S.A./N.V. (“Euroclear”) and their book-entry systems and procedures from sources that we believe to be reliable. We take no responsibility for an accurate portrayal of this information. In addition, the description of the clearing systems in this section reflects our understanding of the rules and procedures of DTC, Clearstream, Luxembourg and Euroclear as they are currently in effect. Those systems could change their rules and procedures at any time.

The Notes will initially be represented by one or more fully registered global notes. Each such global note will be deposited with, or on behalf of, DTC or any successor thereto and registered in the name of Cede & Co. (DTC’s nominee). You may hold your interests in the global notes in the United States through DTC, or in Europe through Clearstream, Luxembourg or Euroclear, either as a participant in such systems or indirectly through organizations which are participants in such systems. Clearstream, Luxembourg and Euroclear will hold interests in the global notes on behalf of their respective participating organizations or customers through customers’ securities accounts in Clearstream, Luxembourg’s or Euroclear’s names on the books of their respective depositaries, which in turn will hold those positions in customers’ securities accounts in the depositaries’ names on the books of DTC.

So long as DTC or its nominee is the registered owner of the global securities representing the Notes, DTC or such nominee will be considered the sole owner and holder of the Notes for all purposes of the Notes and the indenture. Except as provided below, owners of beneficial interests in the Notes will not be entitled to have the Notes registered in their names, will not receive or be entitled to receive physical delivery of the Notes in definitive form and will not be considered the owners or holders of the Notes under the indenture, including for purposes of receiving any reports delivered by the Issuer, Aon plc or the trustee pursuant to the indenture. Accordingly, each person owning a beneficial interest in a Note must rely on the procedures of DTC or its nominee and, if such person is not a participant, on the procedures of the participant through which such person owns its interest, in order to exercise any rights of a holder of Notes.

Unless and until Aon Corporation issues the Notes in fully certificated, registered form under the limited circumstances described below under the heading “—Certificated Notes”:

The Depository Trust Company

DTC will act as securities depositary for the Notes. The Notes will be issued as fully registered Notes registered in the name of Cede & Co. DTC is:

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DTC holds securities that its direct participants deposit with DTC. DTC facilitates the settlement among direct participants of securities transactions, such as transfers and pledges, in deposited securities through electronic computerized book-entry changes in direct participants’ accounts, thereby eliminating the need for physical movement of securities certificates.

Direct participants of DTC include securities brokers and dealers (including the underwriters), banks, trust companies, clearing corporations and certain other organizations. DTC is owned by a number of its direct participants. Indirect participants of DTC, such as securities brokers and dealers, banks and trust companies, can also access the DTC system if they maintain a custodial relationship with a participant, either directly or indirectly.

Purchases of Notes under DTC’s system must be made by or through direct participants, which will receive a credit for the Notes on DTC’s records. The ownership interest of each beneficial owner is in turn to be recorded on the records of direct participants and indirect participants. Beneficial owners will not receive written confirmation from DTC of their purchase, but beneficial owners are expected to receive written confirmations providing details of the transaction, as well as periodic statements of their holdings, from the direct participants or indirect participants through which such beneficial owners entered into the transaction. Transfers of ownership interests in the Notes are to be accomplished by entries made on the books of participants acting on behalf of beneficial owners. Beneficial owners will not receive certificates representing their ownership interests in Notes, except as provided below in “— Certificated Notes.”

To facilitate subsequent transfers, all Notes deposited with DTC are registered in the name of DTC’s nominee, Cede & Co. The deposit of Notes with DTC and their registration in the name of Cede & Co. effect no change in beneficial ownership. DTC has no knowledge of the actual beneficial owners of the Notes. DTC’s records reflect only the identity of the direct participants to whose accounts such Notes are credited, which may or may not be the beneficial owners. The participants will remain responsible for keeping account of their holdings on behalf of their customers.

Conveyance of notices and other communications by DTC to direct participants, by direct participants to indirect participants and by direct participants and indirect participants to beneficial owners will be governed by arrangements among them, subject to any statutory or regulatory requirements as may be in effect from time to time.

Book-Entry Format

Under the book-entry format, the paying agent will pay interest or principal payments to Cede & Co., as nominee of DTC. DTC will forward the payment to the direct participants, who will then forward the payment to the indirect participants (including Clearstream, Luxembourg or Euroclear) or to you as the beneficial owner. You may experience some delay in receiving your payments under this system. Neither we, Aon Corporation, the trustee under the indenture nor any paying agent has any direct responsibility or liability for the payment of principal or interest on the Notes to owners of beneficial interests in the Notes.

DTC is required to make book-entry transfers on behalf of its direct participants and is required to receive and transmit payments of principal, premium, if any, and interest on the Notes. Any direct participant or indirect participant with which you have an account is similarly required to make book-entry transfers and to receive and transmit payments with respect to the Notes on your behalf. We, Aon Corporation and the trustee under the indenture have no responsibility for any aspect of the actions of DTC, Clearstream, Luxembourg or Euroclear or any of their direct or indirect participants. In addition, we, Aon Corporation and the trustee under

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the indenture have no responsibility or liability for any aspect of the records kept by DTC, Clearstream, Luxembourg, Euroclear or any of their direct or indirect participants relating to or payments made on account of beneficial ownership interests in the Notes or for maintaining, supervising or reviewing any records relating to such beneficial ownership interests. We and Aon Corporation also do not supervise these systems in any way.

The trustee will not recognize you as a holder under the indenture, and you can only exercise the rights of a holder indirectly through DTC and its direct participants. DTC has advised us that it will only take action regarding a Note if one or more of the direct participants to whom the Note is credited directs DTC to take such action and only in respect of the portion of the aggregate principal amount of the Notes as to which that participant or participants has or have given that direction. DTC can only act on behalf of its direct participants. Your ability to pledge Notes to non-direct participants, and to take other actions, may be limited because you will not possess a physical certificate that represents your Notes.

Neither DTC nor Cede & Co. (nor such other DTC nominee) will consent or vote with respect to the Notes unless authorized by a direct participant in accordance with DTC’s procedures. Under its usual procedures, DTC will mail an omnibus proxy to us as soon as possible after the record date. The omnibus proxy assigns Cede & Co.’s consenting or voting rights to those direct participants to whose accounts the Notes are credited on the record date (identified in a listing attached to the omnibus proxy).

Clearstream, Luxembourg or Euroclear will credit payments to the cash accounts of Clearstream, Luxembourg customers or Euroclear participants in accordance with the relevant system’s rules and procedures, to the extent received by its depositary. These payments will be subject to tax reporting in accordance with relevant United States tax laws and regulations. Clearstream, Luxembourg or the Euroclear Operator, as the case may be, will take any other action permitted to be taken by a holder under the indenture on behalf of a Clearstream, Luxembourg customer or Euroclear participant only in accordance with its relevant rules and procedures and subject to its depositary’s ability to effect those actions on its behalf through DTC.

DTC, Clearstream, Luxembourg and Euroclear have agreed to the foregoing procedures in order to facilitate transfers of the Notes among participants of DTC, Clearstream, Luxembourg and Euroclear. However, they are under no obligation to perform or continue to perform those procedures, and they may discontinue those procedures at any time.

Transfers Within and Among Book-Entry Systems

Transfers between DTC’s direct participants will occur in accordance with DTC rules. Transfers between Clearstream, Luxembourg customers and Euroclear participants will occur in accordance with its applicable rules and operating procedures.

DTC will effect cross-market transfers between persons holding directly or indirectly through DTC, on the one hand, and directly or indirectly through Clearstream, Luxembourg customers or Euroclear participants, on the other hand, in accordance with DTC rules on behalf of the relevant European international clearing system by its depositary. However, cross-market transactions will require delivery of instructions to the relevant European international clearing system by the counterparty in that system in accordance with its rules and procedures and within its established deadlines (European time). The relevant European international clearing system will, if the transaction meets its settlement requirements, instruct its depositary to effect final settlement on its behalf by delivering or receiving securities in DTC, and making or receiving payment in accordance with normal procedures for same-day funds settlement applicable to DTC. Clearstream, Luxembourg customers and Euroclear participants may not deliver instructions directly to the depositaries.

Because of time-zone differences, credits of securities received in Clearstream, Luxembourg or Euroclear resulting from a transaction with a DTC direct participant will be made during the subsequent securities settlement processing, dated the business day following the DTC settlement date.

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Those credits or any transactions in those securities settled during that processing will be reported to the relevant Clearstream, Luxembourg customer or Euroclear participant on that business day. Cash received in Clearstream, Luxembourg or Euroclear as a result of sales of securities by or through a Clearstream, Luxembourg customer or a Euroclear participant to a DTC direct participant will be received with value on the DTC settlement date but will be available in the relevant Clearstream, Luxembourg or Euroclear cash amount only as of the business day following settlement in DTC.

Although DTC, Clearstream, Luxembourg and Euroclear have agreed to the foregoing procedures in order to facilitate transfers of debt securities among their respective participants, they are under no obligation to perform or continue to perform such procedures and such procedures may be discontinued at any time.

Certificated Notes

Unless and until they are exchanged, in whole or in part, for Notes in definitive form in accordance with the terms of the Notes, the Notes may not be transferred except (1) as a whole by DTC to a nominee of DTC or (2) by a nominee of DTC to DTC or another nominee of DTC or (3) by DTC or any such nominee to a successor of DTC or a nominee of such successor.

Aon Corporation will issue Notes to you or your nominees, in fully certificated registered form, rather than to DTC or its nominees, only if:

If any of the three above events occurs, DTC is required to notify all direct participants that Notes in fully certificated registered form are available through DTC. DTC will then surrender the global notes representing the Notes along with instructions for re-registration. The trustee will re-issue the Notes in fully certificated registered form and will recognize the registered holders of the certificated debt securities as holders under the indenture.

Unless and until Aon Corporation issues the Notes in fully certificated, registered form, (1) you will not be entitled to receive a certificate representing your interest in the Notes; (2) all references in this prospectus supplement or the accompanying prospectus to actions by holders will refer to actions taken by the depositary upon instructions from their direct participants; and (3) all references in this prospectus supplement or the accompanying prospectus to payments and notices to holders will refer to payments and notices to the depositary, as the registered holder of the Notes, for distribution to you in accordance with its policies and procedures.

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UNDERWRITING

Under the terms and subject to the conditions contained in an underwriting agreement dated the date of this prospectus supplement, Aon Corporation and Aon plc have agreed to sell to the underwriters named below, for whom Citigroup Global Markets Inc., Barclays Capital Inc. and Credit Suisse Securities (USA) LLC are acting as representatives, the following respective principal amounts of the Securities.

The underwriting agreement provides that the underwriters are obligated to purchase all of the Securities if any are purchased. The underwriting agreement also provides that if an underwriter defaults with respect to the Securities the purchase commitments of non-defaulting underwriters may be increased or the offering of the Securities may be terminated.

The underwriters propose to offer the Securities initially at the applicable public offering price on the cover page of this prospectus supplement and to selling group members at that price less a selling concession of     % of the principal amount of the Securities. The underwriters may allow, and dealers may re-allow, a concession not to exceed     % of the principal amount of the Securities on sales to other dealers. After the initial public offering the representatives may change the public offering price and concession and discount to broker/dealers.

We estimate that our out of pocket expenses for this offering will be approximately $            .

The Securities are a new issue of securities with no established trading market, and we currently have no intention to apply to list the Securities on any securities exchange or to seek their admission to trading on any automated quotation system. One or more of the underwriters intend(s) to make a secondary market for the Securities. However, they are not obligated to do so and may discontinue making a secondary market for the Securities at any time without notice. No assurance can be given as to how liquid the trading market for the Securities will be.

We have agreed that we will not offer, sell, contract to sell, pledge or otherwise dispose of, directly or indirectly, or file with the Securities and Exchange Commission a registration statement under the Securities Act of 1933, as amended (the “Securities Act”) relating to, any additional debt securities, or publicly disclose the intention to make any such offer, sale, pledge, disposition or filing, without the prior written consent of the representatives until the settlement date for the Securities.

We have agreed to indemnify the several underwriters against liabilities under the Securities Act, or contribute to payments which the underwriters may be required to make in that respect.

In connection with the offering, the underwriters may engage in stabilizing transactions, over-allotment transactions, syndicate covering transactions and penalty bids in accordance with Regulation M under the Exchange Act.

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These stabilizing transactions, syndicate covering transactions and penalty bids may have the effect of raising or maintaining the market price of the Securities or preventing or retarding a decline in the market price of the Securities. As a result the price of the Securities may be higher than the price that might otherwise exist in the open market. These transactions, if commenced, may be discontinued at any time.

Conflicts of Interest; Other Relationships

The underwriters and their respective affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment research, principal investment, hedging, financing and brokerage activities. From time to time in the ordinary course of their respective businesses, certain of the underwriters and their affiliates have engaged in and may in the future engage in commercial banking, derivatives and/or financial advisory, investment banking and other commercial transactions and services with us and our affiliates for which they have received or will receive customary fees and commissions.

In addition, in the ordinary course of their business activities, the underwriters and their affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers. If any of the underwriters or their affiliates have a lending relationship with us, certain of those underwriters or their affiliates routinely hedge, and certain other of those underwriters may hedge, their credit exposure to us consistent with their customary risk management policies. Typically, these underwriters and their affiliates would hedge such exposure by entering into transactions which consist of either the purchase of credit default swaps or the creation of short positions in our securities, including potentially the securities offered hereby. Any such credit default swaps or short positions could adversely affect future trading prices of the securities offered hereby. Such investments and securities activities may involve securities and/or instruments of ours or our affiliates. The underwriters and their affiliates may also make investment recommendations and/or publish or express independent research views in respect of such securities or financial instruments and may hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

Selling Restrictions

Canada

The Securities may be sold only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument 45-106 Prospectus Exemptions or subsection 73.3(1) of the Securities Act (Ontario), and are permitted clients, as defined in National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the Securities must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

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Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus supplement (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 of National Instrument 33-105 Underwriting Conflicts (“NI 33-105”), the underwriters are not required to comply with the disclosure requirements of NI 33-105 regarding underwriter conflicts of interest in connection with this offering.

European Economic Area

This prospectus supplement and the accompanying prospectus have been prepared on the basis that any offer of the Securities in any Member State of the EEA will be made pursuant to an exemption under the Prospectus Directive (as defined below) from the requirement to publish a prospectus for offers of the Securities. Neither this prospectus supplement nor the accompanying prospectus is a prospectus for the purposes of the European Union’s Directive 2003/71/EC (as amended, including by Directive 2010/73/EU, the “Prospectus Directive”), as implemented in the Member States of the EEA.

MiFID II product governance. Solely for the purposes of each manufacturer’s product approval process, the target market assessment in respect of the Securities has led to the conclusion that: (i) the target market for the Securities is eligible counterparties and professional clients only, each as defined in MiFID II; and (ii) all channels for distribution of the Securities to eligible counterparties and professional clients are appropriate. Any distributor should take into consideration the manufacturers’ target market assessment; however, a distributor subject to MiFID II is responsible for undertaking its own target market assessment in respect of the Securities (by either adopting or refining the manufacturers’ target market assessment) and determining appropriate distribution channels. For these purposes “manufacturer” and “distributor” are to be construed in accordance with MiFID II and the rules and regulations promulgated thereunder.

Prohibition of sales to EEA retail investors. The Securities are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the EEA. For these purposes, a retail investor means a person who is one (or more) of: (i) a retail client as defined in point (11) of Article 4(1) of MiFID II; or (ii) a customer within the meaning of the Insurance Distribution Directive, where that customer would not qualify as a professional client as defined in point (1) of Article 4(1) of MiFID II; or (iii) not a qualified investor as defined in the Prospectus Directive. Consequently, no key information document required by the PRIIPs Regulation for offering or selling the Securities or otherwise making them available to retail investors in the EEA has been prepared, and therefore offering and selling the Securities or otherwise making them available to any retail investor in the EEA may be unlawful under the PRIIPs Regulation.

United Kingdom

The communication of this prospectus supplement, the accompanying prospectus and any other document or materials relating to the issue of the Securities offered hereby is not being made, and such documents and/or materials have not been approved, by an authorized person for the purposes of section 21 of the United Kingdom’s Financial Services and Markets Act 2000. Accordingly, such documents and/or materials are not being distributed to, and must not be passed on to, the general public in the United Kingdom. The communication of such documents and/or materials as a financial promotion is only being made to those persons in the United Kingdom falling within the definition of investment professionals (as defined in Article 19 (5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005 (the “Financial Promotion Order”), or within Article 49(2)(a) to (d) of the Financial Promotion Order, or to any other persons to whom it

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may otherwise lawfully be made under the Financial Promotion Order (all such persons together being referred to as “relevant persons”). In the United Kingdom, the Securities offered hereby are only available to, and any investment or investment activity to which this offering circular relates will be engaged in only with, relevant persons. Any person in the United Kingdom that is not a relevant person should not act or rely on this prospectus supplement, the accompanying prospectus or any of the contents thereof.

All applicable provisions of the FSMA with respect to anything done in relation to the Securities in, from or otherwise involving the United Kingdom must be complied with.

Switzerland

The Securities may not be publicly offered in Switzerland and will not be listed on the SIX Swiss Exchange (the “SIX”) or on any other stock exchange or regulated trading facility in Switzerland. This document has been prepared without regard to the disclosure standards for issuance prospectuses under art. 652a or art. 1156 of the Swiss Code of Obligations or the disclosure standards for listing prospectuses under art. 27 ff. of the SIX Listing Rules or the listing rules of any other stock exchange or regulated trading facility in Switzerland. Neither this document nor any other offering or marketing material relating to the Securities or the offering may be publicly distributed or otherwise made publicly available in Switzerland.

Neither this prospectus supplement nor any other offering or marketing material relating to the offering, the issuer, or the Securities have been or will be filed with or approved by any Swiss regulatory authority. In particular, this prospectus supplement will not be filed with, and the offer of Securities will not be supervised by, the Swiss Financial Market Supervisory Authority, and the offer of Securities has not been and will not be authorized under the Swiss Federal Act on Collective Investment Schemes (the “CISA”). The investor protection afforded to acquirers of interests in collective investment schemes under the CISA does not extend to acquirers of Securities.

Hong Kong

This prospectus has not been approved by or registered with the Securities and Futures Commission of Hong Kong or the Registrar of Companies of Hong Kong. The Securities will not be offered or sold in Hong Kong other than (a) to “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or (b) in other circumstances which do not result in the document being a “prospectus” as defined in the Companies Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of that Ordinance. No advertisement, invitation or document relating to the Securities which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do so under the securities laws of Hong Kong) has been issued or will be issued in Hong Kong or elsewhere other than with respect to Securities which are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” as defined in the Securities and Futures Ordinance and any rules made under that Ordinance.

Japan

The Securities have not been and will not be registered under the Financial Instruments and Exchange Law of Japan (Law No. 25 of 1948, as amended) and, accordingly, will not be offered or sold, directly or indirectly, in Japan, or for the benefit of any Japanese Person or to others for re-offering or resale, directly or indirectly, in Japan or to any Japanese Person, except in compliance with all applicable laws, regulations and ministerial guidelines promulgated by relevant Japanese governmental or regulatory authorities in effect at the relevant time. For the purposes of this paragraph, “Japanese Person” shall mean any person resident in Japan, including any corporation or other entity organized under the laws of Japan.

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Singapore

This prospectus has not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the Securities may not be circulated or distributed, nor may the Securities be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act (Chapter 289) (the “SFA”), (ii) to a relevant person, or any person pursuant to Section 275(1A), and in accordance with the conditions, specified in Section 275 of the SFA or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA. Where the Securities are subscribed or purchased under Section 275 by a relevant person which is: (a) a corporation (which is not an accredited investor) the sole business of which is to hold investments and the entire share capital of which is owned by one or more individuals, each of whom is an accredited investor; or (b) a trust (where the trustee is not an accredited investor) whose sole purpose is to hold investments and each beneficiary is an accredited investor, then securities, debentures and units of securities and debentures of that corporation or the beneficiaries’ rights and interest in that trust shall not be transferable for 6 months after that corporation or that trust has acquired the Securities under Section 275 except: (i) to an institutional investor under Section 274 of the SFA or to a relevant person, or any person pursuant to Section 275(1A), and in accordance with the conditions, specified in Section 275 of the SFA; (ii) where no consideration is given for the transfer; or (iii) by operation of law.

Notification under Section 309B(1)(c) of the SFA. The Company has determined that the Securities are (A) prescribed capital markets products (as defined in the Securities and Futures (Capital Markets Products) Regulations 2018) and (B) Excluded Investment Products (as defined in MAS Notice SFA 04-N12: Notice on the Sale of Investment Products and MAS Notice FAA-N16: Notice on Recommendations on Investment Products).

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INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The SEC allows us to “incorporate by reference” into this prospectus supplement the information we file with the SEC, which means that we can disclose important information to you by referring you to those documents. Any information incorporated this way is considered to be part of this prospectus supplement, and any information that we file later with the SEC will automatically update and supersede this information. SEC rules and regulations also allow us to “furnish” rather than “file” certain reports and information with the SEC. Any such reports or information which we have indicated as being “furnished” shall not be deemed to be incorporated by reference in or otherwise become a part of this prospectus, regardless of when furnished to the SEC. We incorporate by reference the following documents that we have filed with the SEC and any future filings that we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, which we refer to as the “Exchange Act” (other than information furnished rather than filed):

You may request a copy of any filing referred to above (including any exhibits that are specifically incorporated by reference), at no cost, by contacting Aon at the following address or telephone number:

LEGAL MATTERS

The validity of the Securities will be passed upon for us as to United States and English law by Latham & Watkins LLP. Davis Polk & Wardwell LLP, New York, New York, will pass upon certain matters for the underwriters.

EXPERTS

The consolidated financial statements of Aon plc appearing in Aon plc’s Annual Report (Form 10-K) for the year ended December 31, 2018, and the effectiveness of Aon plc’s internal control over financial reporting as of December 31, 2018, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon, included therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance on their authority as experts in accounting and auditing.

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PROSPECTUS

Aon plc

Debt Securities

Guarantees

Preference Shares

Class A Ordinary Shares

Share Purchase Contracts

Share Purchase Units

Aon Corporation

Debt Securities

Guarantees

We may from time to time offer and sell any of the securities identified above, in each case, in one or more series and in one or more offerings. This prospectus provides you with a general description of the securities. Our subsidiary, Aon Corporation, which we refer to as “Aon Delaware,” also may from time to time offer and sell its debt securities in one or more series. Aon Delaware may guarantee all payments of principal, interest (if any), premium (if any) and other amounts due on any debt securities we issue. We may guarantee all payments of principal, interest (if any), premium (if any) and other amounts due on any debt securities Aon Delaware issues.

We may offer and sell the securities described in this prospectus and any prospectus supplement to or through one or more underwriters, dealers and agents or directly to purchasers, or through a combination of these methods. If any underwriters, dealers or agents are involved in the sale of any of the securities, their names and any applicable purchase price, fee, commission or discount arrangement between or among them will be set forth, or will be calculable from the information set forth, in the applicable prospectus supplement. See the sections of this prospectus entitled “About this Prospectus” and “Plan of Distribution” for more information. No securities may be sold without delivery of this prospectus and the applicable prospectus supplement describing the method and terms of the offering of such securities.

Each time we or Aon Delaware offer and sell securities, we or Aon Delaware will provide a supplement to this prospectus that contains specific information about the offering and the amounts, prices and terms of the securities. The supplement may also add, update or change information contained in this prospectus with respect to that offering. You should carefully read this prospectus and the applicable prospectus supplement before you invest in any of our securities, as well as the documents incorporated or deemed to be incorporated by reference in this prospectus and the applicable prospectus supplement.

INVESTING IN THE SECURITIES DESCRIBED IN THIS PROSPECTUS INVOLVES RISK. YOU SHOULD CAREFULLY REVIEW THE RISKS AND UNCERTAINTIES DESCRIBED UNDER THE HEADING “RISK FACTORS” BEGINNING ON PAGE 8 OF THIS PROSPECTUS AND ANY RISK FACTORS SET FORTH IN THE APPLICABLE PROSPECTUS SUPPLEMENT AND IN THE DOCUMENTS INCORPORATED OR DEEMED TO BE INCORPORATED BY REFERENCE IN THIS PROSPECTUS OR ANY APPLICABLE PROSPECTUS SUPPLEMENT.

Our executive offices are located at The Aon Centre, The Leadenhall Building, 122 Leadenhall Street, London, England, EC3V 4AN, and our telephone number is +44 20 7623 5500. Aon Delaware’s executive offices are located at 200 East Randolph Street, Chicago, Illinois 60601, and Aon Delaware’s telephone number is (312) 381-1000.

Our Class A Ordinary Shares are listed on the New York Stock Exchange under the symbol “AON.”

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is September 25, 2018.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS

This prospectus is part of an automatic shelf registration statement that we filed with the U.S. Securities and Exchange Commission, or the SEC, as a “well-known seasoned issuer” as defined in Rule 405 under the Securities Act of 1933, as amended, using an automatic “shelf” registration process. By using a shelf registration statement, we may, over time, offer any combination of debt securities, guarantees, preference shares, Class A Ordinary Shares, share purchase contracts and share purchase units described in this prospectus in one or more offerings; and our subsidiary, Aon Delaware, may, over time, offer any combination of debt securities and guarantees described in this prospectus in one or more offerings. In this prospectus we refer to the debt securities, guarantees, preference shares, Class A Ordinary Shares, share purchase contracts and share purchase units offered by us, and the debt securities and guarantees offered by Aon Delaware, collectively as the “securities.” This prospectus provides you with a general description of the securities we or Aon Delaware may offer. Each time that we or Aon Delaware offer and sell securities, we or Aon Delaware will provide a prospectus supplement to this prospectus that contains specific information about the securities being offered and sold and the specific terms of that offering. We may also authorize one or more free writing prospectuses to be provided to you that may contain material information relating to these offerings. The prospectus supplement or free writing prospectus may also add, update or change information contained in this prospectus with respect to that offering. If there is any inconsistency between the information in this prospectus and the applicable prospectus supplement or free writing prospectus, you should rely on the prospectus supplement or free writing prospectus, as applicable. Before purchasing any securities, you should carefully read both this prospectus and the applicable prospectus supplement (and any applicable free writing prospectuses), together with the additional information described under the headings “Where You Can Find More Information” and “Incorporation by Reference.”

As allowed by SEC rules, this prospectus does not contain all the information you can find in the registration statement of which this prospectus is a part or the exhibits to the registration statement. For further information, we refer you to the registration statement of which this prospectus is a part, including its exhibits and schedules. Statements contained in this prospectus about the provisions or contents of any contract, agreement or other document are not necessarily complete. For each of these contracts, agreements or documents filed as an exhibit to the registration statement, we refer you to the actual exhibit for a more complete description of the matters involved. You should rely only on the information contained or incorporated or deemed to be incorporated by reference in this prospectus and each applicable prospectus supplement. We have not authorized anyone else to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. You should not assume that the information contained or incorporated or deemed to be incorporated by reference in this prospectus or any applicable prospectus supplement is accurate as of any date other than the date on the cover of the applicable document. Our, or Aon Delaware’s, business, financial condition and results of operations may have changed since that date. Neither this prospectus nor any prospectus supplement constitutes an offer to sell securities or a solicitation of an offer to buy securities by anyone in any jurisdiction in which that offer or solicitation is not authorized, or in which the person is not qualified to do so or to any person to whom it is unlawful to make that offer or solicitation.

This prospectus is not intended to be and is not a prospectus for purposes of the E.U. Prospectus Directive and/or the U.K. Financial Services Authority’s Prospectus Rules.

Neither we, nor Aon Delaware have authorized anyone to provide you with any information or to make any representations other than those contained, or incorporated by reference, in this prospectus, any applicable prospectus supplement or any free writing prospectuses prepared by or on behalf of us or Aon Delaware or to which we have referred you. We and Aon Delaware take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. We and Aon Delaware will not make an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus and the applicable prospectus supplement to this prospectus is accurate only as of the date on its respective cover, that the information appearing in any applicable free writing prospectus is accurate only as of the date of that free writing prospectus, and that any information incorporated

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by reference is accurate only as of the date of the document incorporated by reference, unless we indicate otherwise. Our business, financial condition, results of operations and prospects may have changed since those dates. This prospectus incorporates by reference, and any prospectus supplement or free writing prospectus may contain and incorporate by reference, market data and industry statistics and forecasts that are based on independent industry publications and other publicly available information. Although we believe these sources are reliable, we do not guarantee the accuracy or completeness of this information and we have not independently verified this information. In addition, the market and industry data and forecasts that may be included or incorporated by reference in this prospectus, any prospectus supplement or any applicable free writing prospectus may involve estimates, assumptions and other risks and uncertainties and are subject to change based on various factors, including those discussed under the heading “Risk Factors” contained in this prospectus, the applicable prospectus supplement and any applicable free writing prospectus, and under similar headings in other documents that are incorporated by reference into this prospectus. Accordingly, investors should not place undue reliance on this information.

When we refer to “Aon plc,” “Aon,” “we,” “our,” “us” and the “Company” in this prospectus, we mean Aon plc, and its subsidiaries, unless otherwise specified. When we refer to “Aon Delaware,” we mean Aon Delaware, our wholly owned subsidiary. In 2012, we reincorporated in the United Kingdom and completed the reorganization of the corporate structure of the group of companies controlled by Aon Delaware, Aon plc’s predecessor as the ultimate holding company of the Aon group. In this prospectus, we refer to this transaction as the “Redomestication.” In the Redomestication, each issued and outstanding share of Aon Delaware common stock held by stockholders of Aon Delaware was converted into the right to receive one Class A Ordinary Share, nominal value $0.01 per share, of Aon plc, subject to the receipt of cash for fractional shares. Any references in this prospectus to “Aon,” the “Company,” “we,” “us” or “our” or any similar references relating to dates or periods before the Redomestication refer to Aon Delaware and its subsidiaries or, if the context so requires, Aon Delaware alone. When we refer to “you,” we mean the potential holders of the applicable series of securities.

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WHERE YOU CAN FIND MORE INFORMATION

We file reports, proxy statements and other information with the SEC. Information filed with the SEC by us can be inspected and copied at the Public Reference Room maintained by the SEC at 100 F Street, N.E., Washington, D.C. 20549. You may also obtain copies of this information by mail from the Public Reference Room of the SEC at prescribed rates. Further information on the operation of the SEC’s Public Reference Room in Washington, D.C. can be obtained by calling the SEC at 1-800-SEC-0330. The SEC also maintains a web site that contains reports, proxy and information statements and other information about issuers, such as us, who file electronically with the SEC. The address of that website is http://www.sec.gov.

Our web site address is http://www.aon.com. The information on our web site, however, is not, and should not be deemed to be, a part of this prospectus.

This prospectus and any prospectus supplement are part of a registration statement that we filed with the SEC and do not contain all of the information in the registration statement. The full registration statement may be obtained from the SEC or us, as provided below. Other documents establishing the terms of the offered securities are or may be filed as exhibits to the registration statement or documents incorporated by reference in the registration statement. Statements in this prospectus or any prospectus supplement about these documents are summaries and each statement is qualified in all respects by reference to the document to which it refers. You should refer to the actual documents for a more complete description of the relevant matters. You may inspect a copy of the registration statement at the SEC’s Public Reference Room in Washington, D.C. or through the SEC’s website, as provided above.

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INCORPORATION BY REFERENCE

The SEC’s rules allow us to “incorporate by reference” information into this prospectus, which means that we can disclose important information to you by referring you to another document filed separately with the SEC. The information incorporated by reference is deemed to be part of this prospectus, and subsequent information that we file with the SEC will automatically update and supersede that information. Any statement contained in this prospectus or a previously filed document incorporated by reference will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained in this prospectus or a subsequently filed document incorporated by reference modifies or replaces that statement.

This prospectus and any accompanying prospectus supplement incorporate by reference the documents set forth below that have previously been filed with the SEC:

All reports and other documents we subsequently file pursuant to Section 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, or the “Exchange Act” in this prospectus, prior to the termination of this offering, including all such documents we may file with the SEC after the date of the initial registration statement and prior to the effectiveness of the registration statement, but excluding any information furnished to, rather than filed with, the SEC, will also be incorporated by reference into this prospectus and deemed to be part of this prospectus from the date of the filing of such reports and documents.

You may request a free copy of any of the documents incorporated by reference in this prospectus by writing or telephoning us at the following address:

Exhibits to the filings will not be sent, however, unless those exhibits have specifically been incorporated by reference in this prospectus or any accompanying prospectus supplement.

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INFORMATION CONCERNING FORWARD LOOKING STATEMENTS

This prospectus, the prospectus supplements, the documents incorporated or deemed to be incorporated by reference in this prospectus or any prospectus supplement and other written or oral statements made from time to time by us may contain certain statements related to future results, or state our intentions, beliefs and expectations or predictions for the future which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. Forward-looking statements relate to expectations or forecasts of future events. They use words such as “anticipate,” “believe,” “estimate,” “expect,” “forecast,” “project,” “intend,” “plan,” “probably,” “potential,” “looking forward” and other similar terms, and future or conditional tense verbs like “could,” “may,” “might,” “should,” “will” and “would.” You can also identify forward-looking statements by the fact that they do not relate strictly to historical or current facts. For example, we may use forward-looking statements when addressing topics such as: market and industry conditions, including competitive and pricing trends; changes in our business strategies and methods of generating revenue; the development and performance of our services and products; changes in the composition or level of our revenues; our cost structure and the outcome of cost-saving or restructuring initiatives; the outcome of contingencies; dividend policy; the expected impact of acquisitions and dispositions; pension obligations; cash flow and liquidity; expected effective tax rate; future actions by regulators; and the impact of changes in accounting rules. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors. Potential factors that could impact results include:

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Any or all of these forward-looking statements may turn out to be inaccurate, and there are no guarantees about our performance. The factors identified above are not exhaustive. We and our subsidiaries operate in a dynamic business environment in which new risks may emerge frequently. Accordingly, you should not place undue reliance on forward-looking statements, which speak only as of the dates on which they are made. We are under no obligation (and expressly disclaim any obligation) to update or alter any forward-looking statement that we may make from time to time, whether as a result of new information, future events or otherwise. Further information about factors that could materially affect us, including our results of operations and financial condition, is contained in the “Risk Factors” section in Part I, Item 1A of our Annual Report on Form 10-K for the year ended December 31, 2017. These factors may be revised or supplemented in subsequent Quarterly Reports on Form 10-Q or Current Reports on Form 8-K.

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RISK FACTORS

Investment in any securities offered pursuant to this prospectus and the applicable prospectus supplement involves risks. You should carefully consider the risk factors incorporated by reference to our most recent Annual Report on Form 10-K and any subsequent Quarterly Reports on Form 10-Q or Current Reports on Form 8-K we file after the date of this prospectus, and all other information contained or incorporated by reference into this prospectus, as updated by our subsequent filings under the Exchange Act, and the risk factors and other information contained in the applicable prospectus supplement and any applicable free writing prospectus before acquiring any of such securities. The occurrence of any of these risks might cause you to lose all or part of your investment in the offered securities.

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THE COMPANY

Aon plc (which may be referred to as “Aon,” “the Company,” “we,” “us,” or “our”) is a leading global professional services firm that provides advice and solutions to clients focused on risk, retirement, and health, delivering distinctive client value via innovative and effective risk management and workforce productivity solutions that are under-pinned by industry-leading data and analytics. Our strategy is to be the preeminent professional service firm in the world, focused on risk and people. Our clients are globally diversified and include all market segments (individuals through personal lines, mid-market companies, and large global companies) and almost every industry in the economy in over 120 countries and sovereignties. This diversification of our customer base helps provide us stability in different economic scenarios that could affect specific industries, customer segments, or geographies. We have continued to focus our portfolio on higher margin, capital-light professional services businesses that have high recurring revenue streams and strong cash flow generation. Aon endeavors to make capital allocation decisions based upon return on invested capital (“ROIC”). On February 9, 2017, the Company entered into a Purchase Agreement with Tempo Acquisition, LLC (the “Purchase Agreement”) to sell its benefits administration and business process outsourcing business (the “Divested Business”) to an entity formed and controlled by affiliates of The Blackstone Group L.P. (the “Buyer”) and certain designated purchasers that are direct or indirect subsidiaries of the Buyer. On May 1, 2017, the Buyer purchased all of the outstanding equity interests of the Divested Business, plus certain related assets and liabilities, for a purchase price of $4.3 billion in cash paid at closing, subject to customary adjustments set forth in the Purchase Agreement, and deferred consideration of up to $500 million. Beginning in the first quarter of 2017 and following the sale of our Divested Business, the Company led a set of initiatives designed to strengthen Aon and unite the firm with one portfolio of capability enabled by proprietary data and analytics and one operating model to deliver additional insight, connectivity, and efficiency. These initiatives reinforce Aon’s ROIC decision-making process and emphasis on free cash flow. The Company is now operating as one segment that includes all of Aon’s continuing operations, which, as a global professional services firm, provides advice and solutions to clients focused on risk, retirement, and health through five principal products and service revenue lines: Commercial Risk Solutions, Reinsurance Solutions, Retirement Solutions, Health Solutions, and Data & Analytic Services. Collectively, these products and service revenue lines make up our one segment: Aon United. As of December 31, 2017, we had approximately 50,000 employees and conducted our operations through various subsidiaries in more than 120 countries and sovereignties. Aon Delaware is an indirect, wholly owned subsidiary of Aon plc. See “About this Prospectus,” “Where You Can Find More Information” and “Incorporation by Reference” in this prospectus.

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USE OF PROCEEDS

Unless we state otherwise in an applicable prospectus supplement, we expect to use the net proceeds from the sale of the securities described in this prospectus and an applicable prospectus supplement for general corporate purposes, including securities repurchase programs, capital expenditures, working capital, repayment or reduction of long term and short term debt and the financing of acquisitions. We may invest funds that we do not immediately require in short term marketable securities.

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RATIOS

The following table sets forth the historical ratios of earnings to fixed charges for Aon plc for the periods indicated.

For this ratio, earnings consist of income from continuing operations before provision for income taxes and noncontrolling interest, less the earnings from unconsolidated entities under the equity method of accounting, and fixed charges. Fixed charges include interest expense and that portion of rental expense we deem to represent interest.

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DESCRIPTION OF DEBT SECURITIES AND GUARANTEES

In this description, references to “Aon,” the “Company,” “we,” “us” or “our” refer only to Aon plc and not to any of our subsidiaries or affiliates, including Aon Delaware. Also, in this section, references to “holders” mean those who own debt securities and the related guarantees registered in their own names, on the books that the appropriate registrar for Aon plc or Aon Delaware, as the case may be, maintains for this purpose, and not those who own beneficial interests in debt securities and the related guarantees registered in “street name” or in debt securities and the related guarantees issued in book-entry form and held through one or more depositaries.

The following description, together with the additional information we include in any applicable prospectus supplement or free writing prospectus, summarizes certain general terms and provisions of the debt securities that we may offer (the “Aon plc debt securities”) or that Aon Delaware may offer (the “Aon Delaware debt securities”) pursuant to this prospectus. When we or Aon Delaware offer to sell a particular series of debt securities, we or Aon Delaware will describe the specific terms of the series in a supplement to this prospectus. We or Aon Delaware will also indicate in the applicable prospectus supplement to what extent the general terms and provisions described in this prospectus apply to a particular series of debt securities.

Aon plc may issue Aon plc debt securities under either (1) a senior indenture (the “Aon plc senior indenture”) among Aon plc, as issuer; Aon Delaware, as guarantor (the “Aon plc senior debt guarantor”) in respect of certain series of Aon plc senior debt securities (as defined below); and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Aon plc senior debt trustee”), (2) a subordinated indenture (the “Aon plc subordinated indenture”) among Aon plc, as issuer; Aon Delaware, as guarantor (the “Aon plc subordinated debt guarantor” and, together with the Aon plc senior debt guarantor, the “Aon plc guarantor”) in respect of certain series of Aon plc subordinated debt securities (as defined below); and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Aon plc subordinated debt trustee” and, together with the Aon plc senior debt trustee, the “Aon plc trustee”), (3) an amended and restated indenture (the “Aon plc A&R Indenture”) among Aon plc, as issuer; the Aon plc Guarantor, as guarantor in respect of a certain series of Aon plc debt securities (as defined below); and the Aon plc trustee, as trustee or (4) an indenture, dated as of November 13, 2015 (the “Aon plc November 2015 Indenture”) among Aon plc, as issuer; the Aon plc Guarantor in respect of a certain series of Aon plc debt securities (as defined below); and the Aon plc trustee, as trustee any Aon plc debt securities that we issue under the Aon plc senior indenture will constitute unsubordinated debt of Aon plc (“Aon plc senior debt securities”) and will rank senior to any Aon plc debt securities that Aon plc issues under the Aon plc subordinated indenture (“Aon plc subordinated debt securities”). Any guarantee that Aon Delaware, as the Aon plc senior debt guarantor, issues under the Aon plc senior indenture will constitute an unsubordinated obligation of Aon Delaware (each, an “Aon plc senior debt guarantee”) and will rank senior to any guarantee that Aon Delaware, as the Aon plc subordinated debt guarantor, issues under the Aon plc subordinated indenture (each, an “Aon plc subordinated debt guarantee” and, together with the Aon plc senior debt guarantees, the “Aon plc debt guarantees”).

Aon Delaware may issue Aon Delaware debt securities under either (1) a senior indenture (the “Aon Delaware senior indenture”) among Aon Delaware, as issuer; Aon plc, as guarantor (the “Aon Delaware senior debt guarantor”) in respect of certain series of Aon Delaware plc senior debt securities (as defined below); and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Aon Delaware senior debt trustee”), or (2) a subordinated indenture (the “Aon Delaware subordinated indenture”) among Aon Delaware, as issuer; Aon plc, as guarantor (the “Aon Delaware subordinated debt guarantor” and, together with the Aon Delaware senior debt guarantor, the “Aon Delaware guarantor”) in respect of certain series of Aon plc Delaware subordinated debt securities (as defined below); and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Aon Delaware subordinated debt trustee” and, together with the Aon Delaware senior debt trustee, the “Aon Delaware trustee”). Any Aon Delaware debt securities that Aon Delaware issues under the Aon Delaware senior indenture will constitute unsubordinated debt of Aon Delaware (“Aon Delaware senior debt securities”) and will rank senior to any Aon Delaware debt securities that Aon Delaware issues under the Aon Delaware subordinated indenture (“Aon Delaware subordinated debt securities”). Any guarantee that Aon plc, as the Aon Delaware

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senior debt guarantor, issues under the Aon Delaware senior indenture will constitute an unsubordinated obligation of Aon plc (each, an “Aon Delaware senior debt guarantee”) and will rank senior to any guarantee that Aon plc, as the Aon Delaware subordinated debt guarantor, issues under the Aon Delaware subordinated indenture (each, an “Aon Delaware subordinated debt guarantee” and, together with the Aon Delaware senior debt guarantees, the “Aon Delaware debt guarantees”).

In this description, the Aon plc debt securities and the Aon Delaware debt securities are sometimes referred to together as the “debt securities,” the Aon plc senior debt securities and the Aon Delaware senior debt securities are sometimes referred to together as the “senior debt securities,” the Aon plc subordinated debt securities and the Aon Delaware subordinated debt securities are sometimes referred together as the “subordinated debt securities,” the Aon plc senior indenture and the Aon Delaware senior indenture are sometimes referred to together as the “senior indentures,” the Aon plc subordinated indenture and the Aon Delaware subordinated indenture are sometimes referred to together as the “subordinated indentures,” the senior indentures and the subordinated indentures are sometimes referred to together as the “indentures,” the Aon plc debt guarantees and the Aon Delaware debt guarantees are sometimes referred to together as the “guarantees,” each of Aon plc and Aon Delaware, in each case in its capacity as issuer of debt securities, is sometimes referred to as an “issuer,” each of the Aon plc debt guarantor and the Aon Delaware debt guarantor is sometimes referred to as a “guarantor,” each of the Aon plc senior trustee and the Aon Delaware senior trustee is sometimes referred to as a “senior trustee,” each of the Aon plc subordinated trustee and the Aon Delaware subordinated trustee is sometimes referred to as a “subordinated trustee,” and each of the senior trustee and the subordinated trustee is sometimes referred to as the “trustee.”

The terms of each series of debt securities will be established by or pursuant to a resolution of our board of directors and set forth or determined in the manner provided in a resolution of our board of directors, in an officers’ certificate or by a supplemental indenture. The terms of any debt securities and, if applicable, the guarantees will include those stated in the applicable indenture and those made part of that indenture by reference to the Trust Indenture Act of 1939, which we refer to as the “Trust Indenture Act.” The debt securities will be subject to all those terms, and we refer prospective purchasers and holders of debt securities and guarantees to the applicable indenture and the Trust Indenture Act for a statement of those terms. The debt securities will be described in a prospectus supplement relating to such series (including any pricing supplement or term sheet).

The following summaries of various provisions of the debt securities, the indentures and the guarantees are not complete. They do not describe certain exceptions and qualifications contained in the debt securities, the indentures and the guarantees, and are qualified in their entirety by reference to the provisions of the debt securities, the indentures and the guarantees. Unless we otherwise indicate, capitalized terms have the meanings assigned to them in the applicable indenture.

An applicable prospectus supplement will specify the issuer, the guarantor, if any, whether the debt securities offered thereby will be senior or subordinated debt and whether the debt securities are to be guaranteed. The debt securities may be issued as part of units consisting of debt securities and other securities that Aon plc or Aon Delaware may offer under this prospectus. If debt securities are issued as part of units of debt securities and other securities that Aon plc or Aon Delaware may issue under this prospectus, an applicable prospectus supplement will describe any applicable material federal income tax consequences to holders.

General

The debt securities will be unsecured obligations of the applicable issuer. None of the indentures limit the amount of debt securities that the issuer may issue. Each indenture provides that the issuer may issue debt securities from time to time in one or more series.

The Aon plc senior debt securities and any Aon Delaware senior debt guarantee will be unsecured and unsubordinated obligations of Aon plc and will rank equally in right of payment with Aon plc’s other unsecured and unsubordinated obligations. The Aon plc subordinated debt securities and any Aon Delaware subordinated

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debt guarantee will be subordinated obligations and will rank junior in right of payment, as more fully described in the applicable subordinated indenture, to Aon plc’s senior indebtedness. Because Aon plc is a holding company, the holders of Aon plc debt securities and Aon Delaware debt guarantees may not receive assets of our subsidiaries in a liquidation or recapitalization until the claims of our subsidiaries’ creditors and any insurance policyholders (in the case of our insurance subsidiaries) are paid, except to the extent that Aon plc may have recognized claims against such subsidiaries. In addition, certain regulatory laws limit some of Aon plc’s subsidiaries from making payments to Aon plc of dividends and on loans and other transfers of funds.

The Aon Delaware senior debt securities and any Aon plc senior debt guarantee will be unsecured and unsubordinated obligations of Aon Delaware and will rank equally in right of payment with Aon Delaware’s other unsecured and unsubordinated obligations. The Aon Delaware subordinated debt securities and any Aon plc subordinated debt guarantee will be subordinated obligations and will rank junior in right of payment, as more fully described in the applicable subordinated indenture, to Aon Delaware’s senior indebtedness. Because Aon Delaware is a holding company, the holders of Aon Delaware debt securities and Aon plc debt guarantees may not receive assets of Aon Delaware’s subsidiaries in a liquidation or recapitalization until the claims of Aon Delaware’s subsidiaries’ creditors and any insurance policyholders (in the case of Aon Delaware’s insurance subsidiaries) are paid, except to the extent that Aon Delaware may have recognized claims against such subsidiaries. In addition, certain regulatory laws limit Aon Delaware’s subsidiaries from making payments to Aon Delaware of dividends and on loans and other transfers of funds.

An applicable prospectus supplement will describe the specific terms relating to the series of debt securities being offered. These terms will include some or all of the following:

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If an issuer denominates the purchase price of a series of debt securities in a non-United States dollar currency or currencies or a non-United States dollar currency unit or units, or if the principal of, any premium and interest on any series of debt securities is payable in a non-United States dollar currency or currencies or a non-United States dollar currency unit or units, an applicable prospectus supplement will describe any special United States federal income tax considerations.

The issuer will pay principal and any interest, premium and additional amounts in the manner, at the places and subject to the restrictions set forth in the applicable debt securities, the applicable indenture and any applicable prospectus supplement. The issuer will not impose a service charge for any transfer or exchange of debt securities, but it may require payment of a sum sufficient to cover any tax or other governmental charge that may be imposed. (Section 2.05 of the indentures)

Unless otherwise indicated in an applicable prospectus supplement, each issuer will issue debt securities in fully registered form, without coupons, in denominations of $1,000 or multiples of $1,000. (Sections 2.01 and 2.04 of the indentures).

The issuer may offer to sell at a substantial discount below their stated principal amount, debt securities bearing no interest or interest at a rate that, at the time of issuance, is below the prevailing market rate. An applicable prospectus supplement will describe any special United States federal income tax considerations applicable to any of those discounted debt securities.

The issuer may offer to sell debt securities in which the principal or interest will be determined by reference to one or more currency exchange rates, commodity prices, equity indices or other factors. The principal amount or payment of interest applicable to those debt securities may be greater than or less than the amount of principal or interest otherwise payable, depending upon the value of the applicable currency, commodity, equity index or other factor on the date on which that principal or interest is due. An applicable prospectus supplement will describe the methods used to determine the amount of principal or interest payable on any date, the currencies, commodities, equity indices or other factors to which the amount payable on that date is linked and certain additional tax considerations applicable to those debt securities.

The indentures do not restrict our or Aon Delaware’s ability to incur unsecured indebtedness or, subject to the restrictions described in “— Consolidation and Merger,” to engage in reorganizations, restructurings, mergers, consolidations or similar transactions that have the effect of increasing our or Aon Delaware’s indebtedness. Accordingly, unless an applicable prospectus supplement states otherwise, neither the debt securities nor any guarantees will contain any provisions that afford holders protection against the issuer or, if applicable, the guarantor incurring unsecured indebtedness or engaging in certain reorganizations or transactions. As a result, we or Aon Delaware could become highly leveraged.

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Events of Default

With respect to any series of debt securities, “event of default” means any of the following:

If there is a continuing event of default with respect to any outstanding series of debt securities, the applicable trustee or the holders of at least 25% of the outstanding principal amount of the debt securities of that series may require the issuer or, if applicable, the guarantor to pay immediately the principal (or, if the debt securities of that series are discount securities, that portion of the principal amount as may be specified in the terms of that series) of and accrued and unpaid interest, if any, on all debt securities of that series. However, at any time after that trustee or the holders, as the case may be, declare that acceleration with respect to debt securities of any series, but before the applicable person has obtained a judgment or decree for payment of the money, the holders of a majority in principal amount of the outstanding debt securities of that series may, under certain conditions, cancel such acceleration if (i) all events of default (other than the non-payment of accelerated principal) with respect to debt securities of that series have been cured or (ii) all such events of default have been waived, each as provided in the applicable indenture. (Section 6.01 of the indentures) For information as to waiver of defaults, see “—Modification and Waiver.” The particular provisions relating to acceleration of the maturity of a portion of the principal amount of such debt securities that are discount securities triggered by an event of default shall be described in an applicable prospectus supplement.

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Each indenture provides that, subject to the duties of the applicable trustee to act with the required standard of care if there is a continuing event of default, the applicable trustee need not exercise any of its rights or powers under the indenture at the request or direction of any of the holders of debt securities, unless those holders have offered to the applicable trustee security or indemnity reasonably satisfactory to it. (Section 7.02 of the indentures) Subject to those provisions for security or indemnification of the applicable trustee and certain other conditions, the holders of a majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the applicable trustee or exercising any trust or power that trustee holds, in each case, with respect to the debt securities of that series. (Section 6.06 of the indentures)

No holder of any debt security of any series will have any right to institute any proceeding with respect to any indenture or for any remedy under the applicable indenture unless:

However, the holder of any debt security will have an absolute and unconditional right to receive payment of the principal of, and any premium or interest on, that debt security on or after the date or dates they are to be paid as expressed in or pursuant to that debt security and to institute suit for the enforcement of any such payment. (Section 6.04 of the indentures)

Each indenture provides that the applicable trustee shall provide notice to the holders of debt securities of any series within 90 days of the occurrence of any default with respect to debt securities of that series known to such trustee, except that the trustee need not provide holders of debt securities of any series notice of any default (other than the non-payment of principal or any premium, interest or additional amounts) if such default has been cured and the applicable trustee considers it in the interest of the holders of debt securities of that series not to provide that notice. (Section 6.07 of the indentures)

Consolidation and Merger

Each indenture provides that each of the issuer and the guarantor may consolidate with or merge or convert into, or convey, transfer or lease its properties or assets substantially as an entirety to, another person without the consent of any debt security holders if, along with certain other conditions set forth in the indentures:

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It is possible that a merger, transfer, lease or other transaction could be treated for United States federal income tax purposes as a taxable exchange by the holders of debt securities or guarantees for new securities, which could result in holders of debt securities or guarantees recognizing taxable gain or loss for US federal income tax purposes. A merger, transfer, lease or other transaction could also have adverse tax consequences to holders of debt securities or guarantees under other tax laws to which the holders are subject.

Payment of Additional Amounts

Payments made by Aon plc, Aon Delaware or a paying agent, as applicable, on the debt securities, or in respect to the guarantees, will be made free and clear of and without withholding or deduction for or on account of any present or future income, stamp or other tax, duty, levy, impost, assessment or other governmental charge of any nature whatsoever imposed or levied by or on behalf of the government of the United Kingdom or the United States (each, a “Home Country Jurisdiction”), of any territory of a Home Country Jurisdiction or by any authority or agency therein or thereof having the power to tax, which we refer to collectively as “Taxes,” unless Aon plc, Aon Delaware or a paying agent is required to withhold or deduct Taxes by law.

If Aon plc, Aon Delaware or a paying agent is required to withhold or deduct any amount for or on account of Taxes from any payment made with respect to the debt securities or the guarantees, Aon plc or Aon Delaware, as applicable, will pay such additional amounts as may be necessary so that the net amount received by each beneficial owner (including additional amounts) after such withholding or deduction will not be less than the amount the beneficial owner would have received if the Taxes had not been withheld or deducted; provided that no additional amounts will be payable with respect to Taxes:

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nor shall additional amounts be paid with respect to any payment of the principal of or premium, if any, or interest, if any, on any debt security or any payment in respect of any guarantee to any such holder or beneficial owner who is a fiduciary or a partnership or a beneficial owner who is other than the sole beneficial owner of such payment to the extent a beneficiary or settlor with respect to such fiduciary or a member of such partnership or a beneficial owner would not have been entitled to such additional amounts had it been the holder of the debt security.

All references in this prospectus, other than under “—Defeasance” below, to the payment of the principal of or premium, if any, or interest, if any, on or the net proceeds received on the sale or exchange of, any debt securities or any payment in respect of any guarantee shall be deemed to include additional amounts to the extent that, in that context, additional amounts are, were or would be payable.

Aon plc has agreed in the indentures that if it maintains a paying agent with respect to a particular series of Aon plc debt securities in any member state of the European Union, it will maintain a paying agent in at least one member state that will not be obliged to withhold or deduct taxes pursuant to European Council Directive 2003/48/EC or any other Directive amending, supplementing or replacing such Directive or any law implementing or complying with, or introduced in order to conform to such Directive or Directives, provided there is at least one member state that does not require a paying agent to withhold or deduct pursuant to such Directive.

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Aon plc’s and Aon Delaware’s obligations to pay additional amounts if and when due will survive the termination of the indentures and the payment of all other amounts in respect of the debt securities.

If, as a result of Aon plc’s or Aon Delaware’s consolidation, merger with or conversion into a successor person organized under the laws of a jurisdiction other than the United Kingdom or the United States (or, in each case, any political subdivision or taxing authority thereof) as described under “—Consolidation and Merger” above, or the conveyance, transfer or lease by Aon plc or Aon Delaware of its assets substantially as an entirety to such successor person, and such an entity expressly assumes the obligations of Aon plc or Aon Delaware under the indentures and any outstanding debt securities or guarantees, as applicable, such successor person will pay additional amounts on the same basis as described above, except that references to a “Home Country Jurisdiction” will be treated as references to the United Kingdom, the United States and the country in which such successor person is organized or resident (or deemed resident for tax purposes).

Optional Tax Redemption

The issuer may redeem any series of debt securities in whole, but not in part, at its option at any time prior to maturity, upon the giving of not less than 30 nor more than 90 days’ notice of tax redemption to the holders, at a redemption price equal to the principal amount plus accrued and unpaid interest, if any, to the redemption date (except in the case of discounted debt securities, which may be redeemed at the redemption price specified by the terms of each series of such debt securities), if:

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No notice of any such redemption may be given earlier than 90 days prior to the earliest date on which Aon plc, Aon Delaware or such successor person, as applicable, would be obligated to pay any additional amounts.

Aon plc, Aon Delaware or such successor person will also pay to each holder, or make available for payment to each such holder, on the redemption date, any additional amounts (as described under “—Payment of Additional Amounts” above) resulting from the payment of such redemption price by it. Prior to the delivery of any notice of redemption, Aon plc, Aon Delaware or such successor person will deliver to the trustee an officer’s certificate stating that it is entitled to effect or cause a redemption and setting forth a statement of facts showing that the conditions precedent of the right so to redeem or cause such redemption have occurred, and in the case of a redemption based on an opinion of independent counsel referred to in the second bullet above, such independent counsel’s opinion. Delivery of any notice of redemption will be conclusive and binding on the holders of the securities being redeemed.

Any notice of redemption will be irrevocable once an officer’s certificate has been delivered to the trustee.

Defeasance

Defeasance and Discharge. Unless the debt securities of any series provide otherwise, the issuer and, if applicable, the guarantor may be discharged from any and all obligations in respect of the debt securities of that series and any related guarantee, as applicable (except for certain obligations to register the transfer or exchange of debt securities of that series, to replace stolen, destroyed, lost or mutilated debt securities of that series, to maintain paying agencies, to execute and furnish definitive securities evidenced by temporary securities, to return moneys deposited with or paid to the trustee or any paying agent remaining unclaimed for three years, to compensate and indemnify the applicable trustee or to furnish such trustee (if that trustee is not the registrar) with the names and addresses of holders of debt securities of that series). This discharge, referred to as defeasance, will occur only if, among other things:

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That opinion must state that the issuer or, if applicable, the guarantor has received from the United States Internal Revenue Service a ruling or, since the date of execution of the applicable indenture, there has been a change in the applicable United States federal income tax law, in any case, in support of that opinion. (Sections 13.02 and 13.04 of the indentures)

In addition, the issuer or, if applicable, the guarantor or both may also obtain a discharge of either indenture with respect to all debt securities issued under that indenture and any related guarantee, as applicable, by depositing with the applicable trustee, in trust, enough money to pay all amounts due on the debt securities on the date those payments are due or upon redemption of all of those debt securities, so long as those debt securities are by their terms to become due and payable within one year or are to be called for redemption within one year. (Section 12.01 of the indentures)

Defeasance of Certain Covenants and Certain Events of Default. Unless the debt securities of any series provide otherwise, upon compliance with certain conditions:

The conditions include, among other things:

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Covenant Defeasance and Certain Other Events of Default. If the issuer or, if applicable, the guarantor exercises or both exercise the option to effect a covenant defeasance with respect to the debt securities of any series as described above and the debt securities of that series are thereafter declared due and payable because of an event of default (other than an event of default caused by failing to comply with the covenants that are defeased), the amount of money and securities it has or they have deposited with the applicable trustee would be sufficient to pay amounts due on the debt securities of that series on their respective due dates but may not be sufficient to pay amounts due on the debt securities of that series at the time of acceleration resulting from that event of default. However, the issuer and, if applicable, the guarantor would remain liable for any shortfall.

Modification and Waiver

Each indenture provides that the issuer and, if applicable, the guarantor may enter into supplemental indentures with the applicable trustee without the consent of the holders of debt securities to:

The issuer and, if applicable, the guarantor may enter into a supplemental indenture to modify an indenture with the consent of the applicable trustee and the holders of at least a majority in principal amount of outstanding debt securities of each series affected by such supplemental indenture. However, the issuer and, if applicable, the guarantor may not modify an indenture without the consent of the holders of all then-outstanding debt securities of the affected series issued under that indenture to:

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The holders of at least a majority in principal amount of the outstanding debt securities of any series may on behalf of the holders of all debt securities of that series waive any past default under the applicable indenture with respect to that series, except a default in the payment of the principal of or any premium or any interest on, any debt security of that series or in respect of a provision which under the applicable indenture cannot be modified or amended without the consent of the holder of each outstanding debt security of that affected series. (Section 6.09 of the indentures)

Global Securities

The debt securities of a series may be issued in whole or in part in the form of one or more global certificates that the issuer will deposit with a depositary identified in an applicable prospectus supplement. Unless and until it is exchanged in whole or in part for the individual debt securities that it represents, a global security may not be transferred except as a whole:

An applicable prospectus supplement will describe the specific terms of the depositary arrangement with respect to a series of debt securities. We anticipate that the following provisions will generally apply to depositary arrangements.

When a global security is issued, the depositary for the global security or its nominee will credit, on its book-entry registration and transfer system, the respective principal amounts of the individual debt securities represented by that global security to the accounts of persons that have accounts with the depositary (“participants”). Those accounts will be designated by the dealers, underwriters or agents with respect to the underlying debt securities or by the issuer if those debt securities are offered and sold directly by the issuer. Ownership of beneficial interests in a global security will be limited to participants or persons that may hold interests through participants. For interests of participants, ownership of beneficial interests in the global security will be shown on records maintained by the applicable depositary or its nominee. For interests of persons other than participants, that ownership information will be shown on the records of participants. Transfer of that ownership will be effected only through those records. The laws of some states require that certain purchasers of securities take physical delivery of securities in definitive form. These limits and laws may impair our ability to transfer beneficial interests in a global security.

As long as the depositary for a global security, or its nominee, is the registered owner of that global security, the depositary or nominee will be considered the sole owner or holder of the debt securities represented by the global

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security for all purposes under the applicable indenture. Except as provided below, owners of beneficial interests in a global security:

Payments of the principal of, any premium on and any interest on individual debt securities represented by a global security registered in the name of a depositary or its nominee will be made to the depositary or its nominee as the registered owner of the global security representing such debt securities. No issuer, guarantor, trustee, paying agent or registrar for the debt securities will be responsible for any aspect of the records relating to or payments made by the depositary or any participants on account of beneficial interests in the global security.

It is expected that the depositary or its nominee, upon receipt of any payment of principal, any premium or interest relating to a global security representing any series of debt securities, immediately will credit participants’ accounts with the payments. Those payments will be credited in amounts proportional to the respective beneficial interests of the participants in the principal amount of the global security as shown on the records of the depositary or its nominee. It is also expected that payments by participants to owners of beneficial interests in the global security held through those participants will be governed by standing instructions and customary practices. This is now the case with securities held for the accounts of customers registered in “street name.” Those payments will be the sole responsibility of those participants.

If the depositary for a series of debt securities is at any time unwilling, unable or ineligible to continue as depositary and a successor depositary is not appointed within 90 days, the issuer will issue individual debt securities of that series in exchange for the global security or securities representing that series. In addition, the issuer may at any time in its sole discretion determine not to have any debt securities of a series represented by one or more global securities. In that event, the issuer will issue individual debt securities of that series in exchange for the global security or securities. Furthermore, if specified in an applicable prospectus supplement, an owner of a beneficial interest in a global security may, on terms acceptable to the issuer, the trustee and the applicable depositary, receive individual debt securities of that series in exchange for those beneficial interests. The foregoing is subject to any limitations described in an applicable prospectus supplement. In any such instance, the owner of the beneficial interest will be entitled to physical delivery of individual debt securities equal in principal amount to the beneficial interest and to have the debt securities registered in its name. Those individual debt securities will be issued in any authorized denominations.

Subordination under the Aon plc Subordinated Debt Indenture

The Aon plc subordinated debt securities and the Aon plc subordinated debt guarantees will be subordinate and junior in right of payment to all senior indebtedness of Aon plc and Aon Delaware, respectively, to the extent provided in the Aon plc subordinated debt indenture. (Sections 15.04 and 16.01 of the Aon plc subordinated debt indenture) Neither Aon plc, as issuer, nor Aon Delaware, as Aon plc subordinated debt guarantor, may make any payments on account of principal or any premium, redemption, interest or any other amount payable under the Aon plc subordinated debt securities or the Aon plc subordinated debt guarantees, as the case may be, at any time when it has defaulted with respect to payment of principal or any premium, interest, sinking fund or other payment due on its senior indebtedness. (Sections 15.05 and 16.02 of the Aon plc subordinated debt indenture) If either Aon plc, as issuer, or Aon Delaware, as Aon plc subordinated debt guarantor, makes any payment described in the foregoing sentence before all of its senior indebtedness is paid in full, such payment or distribution will be applied to pay off the applicable senior indebtedness which remains unpaid. Subject to the

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condition that the senior indebtedness of Aon plc or Aon Delaware, as the case may be, is paid in full, if any such payments are made on the senior indebtedness of Aon plc or Aon Delaware, as the case may be, as described above, the holders of Aon plc subordinated debt securities or Aon plc subordinated debt guarantees will be subrogated to the rights of the senior debt security holders of Aon plc or Aon Delaware, as the case may be. (Sections 15.6 and 16.03 of the Aon plc subordinated debt indenture).

The Aon plc subordinated debt indenture defines the term “senior indebtedness” to mean:

“Senior indebtedness” does not include:

Because of the subordination provisions described above, some of the general creditors of Aon plc or Aon Delaware, as the case may be, may recover proportionately more than holders of the Aon plc subordinated debt securities or Aon plc subordinated debt guarantees if the assets of Aon plc or Aon Delaware, as the case may be, are distributed as a result of insolvency or bankruptcy. The Aon plc subordinated debt indenture provides that the subordination provisions will not apply to cash, properties and securities held in trust pursuant to the satisfaction and discharge and the legal defeasance provisions of the Aon plc subordinated debt indenture. (Section 16.03 of the Aon plc subordinated debt indenture) See “—Defeasance” for additional information regarding the legal defeasance provisions affecting the subordinated debt.

We will set forth (or incorporate by reference) the approximate amount of senior indebtedness outstanding for each of Aon plc and Aon Delaware as of a recent date in any prospectus supplement under which we offer to sell Aon plc subordinated debt securities.

Subordination under the Aon Delaware Subordinated Debt Indenture

The Aon Delaware subordinated debt securities and the Aon Delaware subordinated debt guarantees will be subordinate and junior in right of payment to all senior indebtedness of Aon Delaware and Aon plc, respectively,

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to the extent provided in the Aon Delaware subordinated debt indenture. (Sections 15.05 and 16.01 of the Aon Delaware subordinated debt indenture) Neither Aon Delaware, as issuer, nor Aon plc, as Aon Delaware subordinated debt guarantor, may make any payments on account of principal or any premium, redemption, interest or any other amount payable under the Aon Delaware subordinated debt securities or the Aon Delaware subordinated debt guarantees, as the case may be, at any time when it has defaulted with respect to payment of principal or any premium, interest, sinking fund or other payment due on its senior indebtedness. (Sections 15.06 and 16.02 of the Aon Delaware subordinated debt indenture) If either Aon Delaware, as issuer, or Aon plc, as Aon Delaware subordinated debt guarantor, makes any payment described in the foregoing sentence before all of its senior indebtedness is paid in full, such payment or distribution will be applied to pay off the applicable senior indebtedness which remains unpaid. Subject to the condition that the senior indebtedness of Aon Delaware or Aon plc, as the case may be, is paid in full, if any such payments are made on the senior indebtedness of Aon Delaware or Aon plc, as the case may be, as described above, the holders of Aon Delaware subordinated debt securities or Aon Delaware subordinated debt guarantees will be subrogated to the rights of the senior debt security holders of Aon Delaware or Aon plc, as the case may be. (Sections 15.07 and 16.03 of the Aon Delaware subordinated debt indenture).

The Aon Delaware subordinated debt indenture defines the term “senior indebtedness” to mean:

“Senior indebtedness” does not include:

Because of the subordination provisions described above, some of the general creditors of Aon Delaware or Aon plc, as the case may be, may recover proportionately more than holders of the Aon Delaware subordinated debt securities or Aon Delaware subordinated debt guarantees if the assets of Aon plc or Aon Delaware, as the case may be, are distributed as a result of insolvency or bankruptcy. The Aon Delaware subordinated debt indenture provides that the subordination provisions will not apply to cash, properties and securities held in trust pursuant

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to the satisfaction and discharge and the legal defeasance provisions of the Aon Delaware subordinated debt indenture. (Section 16.03 of the Aon Delaware subordinated debt indenture) See “—Defeasance” for additional information regarding the legal defeasance provisions affecting the subordinated debt.

Aon Delaware will set forth (or incorporate by reference) the approximate amount of senior indebtedness outstanding for each of Aon Delaware and Aon plc as of a recent date in any prospectus supplement under which Aon Delaware offers to sell Aon Delaware subordinated debt securities.

Guarantees

Under each guarantee, the applicable guarantor will unconditionally guarantee the due and punctual payment of the principal, interest (if any), premium (if any) and all other amounts due on the applicable debt securities and under the indenture when the same shall become due and payable, whether at maturity, pursuant to mandatory or optional redemption or repayments, by acceleration or otherwise, in each case after any applicable grace periods or notice requirements, according to the terms of the applicable debt securities.

The obligations of each guarantor under the guarantees will be unconditional, regardless of the enforceability of the applicable debt securities, and will not be discharged until all obligations under those debt securities and the applicable indenture are satisfied. Holders of the applicable debt securities may proceed directly against the guarantor under the applicable guarantee if an event of default affecting those debt securities occurs without first proceeding against the issuer.

Conversion Rights

An applicable prospectus supplement will describe the terms and conditions, if any, on which debt securities being offered are convertible into Class A Ordinary Shares or other securities. Such terms will include the conversion price, the conversion period, provisions as to whether conversion will be at the issuer’s option or the option of the holder, the events requiring an adjustment of the conversion price and provisions affecting conversion in the event of the redemption of those debt securities.

Regarding the Trustee

The issuers have commercial deposits and custodial arrangements with The Bank of New York Mellon Trust Company, N.A., or “BNYM,” and may have borrowed money from BNYM in the normal course of business. The issuers may enter into similar or other banking relationships with BNYM in the future in the normal course of business. In addition, the issuers have provided brokerage and other insurance services in the ordinary course of their respective businesses for BNYM. BNYM may also act as trustee with respect to other debt securities one or both of the issuers have issued.

BNYM will be serving as the trustee under the senior indentures and the subordinated indentures. Consequently, if an actual or potential event of default occurs with respect to either the senior debt securities or the subordinated debt securities, BNYM may be considered to have a conflicting interest for purposes of the Trust Indenture Act. In that case, BNYM may be required to resign under one or more indentures, and the applicable issuer and, if applicable, the applicable guarantor would be required to appoint a successor trustee. For this purpose, a “potential” event of default means an event that would be an event of default if the requirements for giving us or Aon Delaware default notice or for the default having to exist for a specific period of time were disregarded.

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Governing Law

The debt securities, the guarantees and the indentures will be governed by and construed in accordance with the laws of the State of New York, except that, as the indentures specify, the subordination provisions of each series of Aon plc subordinated debt securities, any Aon Delaware subordinated debt guarantee and the Aon plc subordinated indenture, as they apply to Aon plc, will be governed by and construed in accordance with the laws of England and Wales (other than the appointment of any attorney-in-fact, which shall be governed by the laws of the State of New York).

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DESCRIPTION OF PREFERENCE SHARES

In this description, references to “Aon,” the “Company,” “we,” “us” or “our” refer only to Aon plc and not to any of our subsidiaries or affiliates. Also, in this section, references to “holders” mean those who own preference shares registered in their own names, on the books that our registrar or we maintain for this purpose, and not those who own beneficial interests in preference shares registered in “street name” or in shares issued in book-entry form and held through one or more depositaries.

The description set forth below is only a summary and is not complete. For more information regarding the preference shares which may be offered by this prospectus, please refer to the documents incorporated by reference in this prospectus, the applicable prospectus supplement or supplements, our articles of association (“Articles”), which are incorporated by reference as an exhibit to the registration statement of which this prospectus forms a part, and any certificate of designations or other instrument establishing a series of preference shares, which will be filed with the SEC as an exhibit to or incorporated by reference in the registration statement at or prior to the time of the issuance of that series of preference shares.

Our Articles authorize us to issue preference shares, in one or more series, with the number of shares of each series and the rights, preferences and limitations of each series to be determined by or pursuant to the authorization of our board of directors at the time of issue of the relevant preference shares. Our board of directors has been generally and unconditionally authorized subject to the provisions of our Articles and the UK Companies Act 2006 (as amended, the “Companies Act”), to allot and issue shares of the Company (or grant rights to subscribe for or to convert any security into shares in the Company), including preference shares, (1) up to an aggregate nominal amount of $808,000 and (2) in connection with an offer of equity securities (as defined in the Companies Act) by way of a rights issue, up to a further aggregate nominal amount of $808,000, all of which remain authorized for allotment and issuance. Our board of directors is also generally empowered to allot equity securities (as defined in the Companies Act) referred to above for cash, free from UK statutory pre-emption rights, provided that, in relation to (1) above, this power is limited to an aggregate nominal amount of $245,000. The authority and the power referred to above will both expire on the earlier of the next annual general meeting of shareholders of the Company or August 31, 2019. Our preference shares will likely be afforded preferences regarding dividends and liquidation rights over our Class A Ordinary Shares and may be issued as redeemable shares, with all such other rights, terms and conditions to be determined by or pursuant to the authorization of the board of directors of Aon plc at the time of issue of the relevant preference shares. If the relevant series of preference shares (with respect to both dividends and capital) are determined to have the right to participate only up to a specified amount in a distribution, they will not constitute “equity securities” for the purposes of the Companies Act.

We will include the specific terms of each series of the preference shares being offered in an applicable prospectus supplement.

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DESCRIPTION OF CLASS A ORDINARY SHARES

In this description, references to “Aon,” the “Company,” “we,” “us” or “our” refer only to Aon plc and not to any of our subsidiaries or affiliates. Also, in this section, references to “holders” mean those who own ordinary shares of Aon registered in their own names, on the books that the registrar or we maintain for this purpose, and not those who own beneficial interests in ordinary shares registered in street name or in shares issued in book-entry form and held through one or more depositaries.

The description of our Class A Ordinary Shares is incorporated into this prospectus by reference to the Current Report on Form 8-K12B filed with the SEC on April 2, 2012. Our Class A Ordinary Shares are listed on the New York Stock Exchange under the symbol “AON”.

For more information regarding the rights attached to the Class A Ordinary Shares which may be offered by this prospectus, please refer to the applicable prospectus supplement or supplements, our Articles, which are incorporated by reference as an exhibit to the registration statement of which this prospectus forms a part, and any other instrument relating to our ordinary shares filed with the SEC as an exhibit to or incorporated by reference in the registration statement at or prior to the time of the issuance of our Class A Ordinary Shares.

Our Articles authorize us to allot and issue Class A Ordinary Shares, and to grant rights to subscribe for or to convert or exchange any security into or for Class A Ordinary Shares, which shall have the same rights, preferences and limitations as the existing Class A Ordinary Shares. Our board of directors has been generally and unconditionally authorized subject to the provisions of our Articles and the Companies Act, to allot and issue shares of the Company (or grant rights to subscribe for or to convert any security into shares in the Company), including Class A Ordinary Shares, up to an aggregate nominal amount of $808,000 and, in connection with an offer of equity securities (as defined in the Companies Act, which would include the Class A Ordinary Shares) by way of a rights issue, up to a further aggregate nominal amount of $808,000, all of which remain authorized for allotment and issuance. Of this amount, all shares remain authorized for allotment and issuance. Our board of directors is also generally empowered to allot equity securities (as defined in the Companies Act) referred to above for cash free from UK statutory pre-emption rights, provided that, in relation to (1) above, this power is limited to an aggregate nominal amount of $245,000. The authority and the power referred to above will both expire on the earlier of the next annual general meeting of shareholders of the Company or August 31, 2019.

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DESCRIPTION OF SHARE PURCHASE CONTRACTS AND SHARE PURCHASE UNITS

In this description, references to “Aon,” the “Company,” “we,” “us” or “our” refer only to Aon plc and not to any of our subsidiaries. We may issue share purchase contracts, representing contracts obligating holders to purchase from us, and obligating us to sell to the holders, a specified number of our Class A Ordinary Shares at a future date or dates. The price per share and the number of our Class A Ordinary Shares may be fixed at the time the share purchase contracts are issued or may be determined by reference to a specific formula set forth in the share purchase contracts. The share purchase contracts may be issued separately or as a part of share purchase units consisting of a share purchase contract and, as security for the holder’s obligations to purchase our Class A Ordinary Shares under our Class A Ordinary Shares purchase contracts:

The share purchase contracts may require us to make periodic payments to the holders of the share purchase units or vice versa, and such payments may be unsecured or prefunded on some basis. The share purchase contracts may require holders to secure their obligations in a specified manner and, in certain circumstances, we may deliver newly issued prepaid share purchase contracts upon release to a holder of any collateral securing such holder’s obligations under the original share purchase contract.

An applicable prospectus supplement will describe the terms of any share purchase contracts or share purchase units and, if applicable, prepaid share purchase contracts.

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PLAN OF DISTRIBUTION

We or Aon Delaware may sell the securities covered by this prospectus in any of the following ways (or in any combination):

Any such dealer or agent, in addition to any underwriter, may be deemed to be an underwriter within the meaning of the Securities Act. Any discounts or commissions received by an underwriter, dealer, remarketing firm or agent on the sale or resale of securities may be considered by the SEC to be underwriting discounts and commissions under the Securities Act.

In addition, we or Aon Delaware may enter into derivative transactions with third parties, or sell securities not covered by this prospectus to third parties in privately negotiated transactions. If an applicable prospectus supplement so indicates, in connection with such a transaction, the third parties may, pursuant to this prospectus and that applicable prospectus supplement, sell securities covered by this prospectus and that applicable prospectus supplement. If so, the third party may use securities borrowed from us or Aon Delaware or others to settle such sales and may use securities received from us to close any related short positions. We or Aon Delaware may also loan or pledge securities covered by this prospectus and an applicable prospectus supplement to third parties, who may sell the loaned securities or, in an event of default in the case of a pledge, sell the pledged securities covered by this prospectus and that applicable prospectus supplement.

The terms of the offering of the securities with respect to which this prospectus is being delivered will be set forth in the applicable prospectus supplement or supplements and will include, among other things:

If underwriters are used in the sale of securities, those securities will be acquired by the underwriters for their own account and may be resold from time to time in one or more transactions, including negotiated transactions,

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at a fixed public offering price or at varying prices determined at the time of sale. The securities may be offered to the public either through underwriting syndicates represented by managing underwriters or directly by one or more underwriters acting alone. Unless otherwise set forth in an applicable prospectus supplement, the obligations of the underwriters to purchase the securities described in an applicable prospectus supplement will be subject to certain conditions precedent, and the underwriters will be obligated to purchase all of those securities if any are purchased by them. Any public offering price and any discounts or concessions allowed or reallowed or paid to dealers may be changed from time to time.

If the dealers acting as principals are used in the sale of any securities, those securities will be acquired by the dealers, as principals, and may be resold from time to time in one or more transactions at varying prices to be determined by the dealer at the time of resale. The name of any dealer and the terms of the transactions will be set forth in an applicable prospectus supplement with respect to the securities being offered.

Securities may also be offered and sold, if so indicated in an applicable prospectus supplement in connection with a remarketing upon their purchase, in accordance with a redemption or repayment pursuant to their terms, or otherwise, by one or more firms, which we refer to as the “remarketing firms,” acting as principals for their own accounts or as our agents, as applicable. Any remarketing firm will be identified and the terms of its agreement, if any, with us or Aon Delaware and its compensation will be described in an applicable prospectus supplement. Remarketing firms may be deemed to be underwriters, as that term is defined in the Securities Act in connection with the securities remarketed thereby.

The securities may be sold directly by us or Aon Delaware or through agents designated by us from time to time. In the case of securities sold directly by us or Aon Delaware, no underwriters or agents would be involved. Any agents involved in the offer or sale of the securities in respect of which this prospectus is being delivered, and any commissions payable by us or Aon Delaware to such agents, will be set forth in an applicable prospectus supplement. Unless otherwise indicated in an applicable prospectus supplement, any such agent will be acting on a best efforts basis for the period of its appointment.

We or Aon Delaware may authorize agents, underwriters or dealers to solicit offers by certain specified institutions to purchase the securities to which this prospectus and the applicable prospectus supplement or supplements relate from us at the public offering price set forth in an applicable prospectus supplement plus, if applicable, accrued interest, pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. Those contracts will be subject only to those conditions set forth in an applicable prospectus supplement, and an applicable prospectus supplement will set forth the commission payable for solicitation of those contracts.

Agents, dealers, underwriters and remarketing firms may be entitled, under agreements entered into with us, Aon Delaware or both, to indemnification by us, Aon Delaware or both against certain civil liabilities, including liabilities under the Securities Act, or to contribution to payments they may be required to make in respect thereof. Agents, dealers, underwriters and remarketing firms may be customers of, engage in transactions with, or perform services for us or our subsidiaries in the ordinary course of business.

Unless otherwise indicated in an applicable prospectus supplement, all securities offered by this prospectus, other than our Class A Ordinary Shares, which are listed on the New York Stock Exchange, will be new issues with no established trading market. We or Aon Delaware, as applicable, may elect to list any of the securities on one or more exchanges, but unless otherwise specified in an applicable prospectus supplement, neither we nor Aon Delaware shall be obligated to do so. In addition, underwriters will not be obligated to make a market in any securities. No assurance can be given regarding the activity of trading in, or liquidity of, any securities.

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Any underwriter may engage in over-allotment, stabilizing transactions, short-covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. Over-allotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying securities so long as the stabilizing bids do not exceed a specified maximum. Short covering transactions involve purchases of the securities in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.

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VALIDITY OF SECURITIES

Latham & Watkins LLP will be requested to advise us with respect to the validity under New York law and English law of any securities that may be offered pursuant to this prospectus and one or more applicable prospectus supplements.

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EXPERTS

The consolidated financial statements of Aon plc appearing in Aon plc’s Annual Report (Form 10-K) for the year ended December 31, 2017, and the effectiveness of Aon plc’s internal control over financial reporting as of December 31, 2017, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon, included therein, and incorporated herein by reference. Such consolidated financial statements as of December 31, 2017 are incorporated herein by reference in reliance on their report given on their authority as experts in accounting and auditing.

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Aon Corporation

$     % Senior Notes due

With a full and unconditional guarantee as to payment of

principal and interest by Aon plc

PROSPECTUS SUPPLEMENT

Citigroup

Barclays

Credit Suisse

                    , 2019