SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE TO/A

(Amendment No. 2)

Tender Offer Statement Under Section 14(d)(1) or 13(e)(1)

of the Securities Exchange Act of 1934

TESARO, INC.

(Name of Subject Company (Issuer))

ADRIATIC ACQUISITION CORPORATION,

GLAXOSMITHKLINE LLC

and

GLAXOSMITHKLINE PLC

(Names of Filing Persons (Offerors))

Common Stock, par value $0.0001 per share

(Title of Class of Securities)

881569107

(CUSIP Number of Class of Securities)

Lisa DeMarco, Esq.

GlaxoSmithKline

1250 S Collegeville Road/ UP4110

Collegeville, Pennsylvania 19426-0989

Telephone: +1 (610) 917-5045

(Name, Address and Telephone Number of Person Authorized

to Receive Notices and Communications on Behalf of Filing Persons)

Copy to:

George A. Casey

George Karafotias

Derrick Lott

Shearman & Sterling LLP

599 Lexington Avenue

New York, NY 10022

Telephone: +1 (212) 848-4000

Calculation of Filing Fee

Check the appropriate boxes below to designate any transactions to which the statement relates:

Check the following box if the filing is a final amendment reporting the results of the tender offer:  ☐

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

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This Amendment No. 2 filed with the Securities and Exchange Commission on December 18, 2018, amends and supplements the Tender Offer Statement filed on Schedule TO (as amended or supplemented, the “Schedule TO”) with the Securities and Exchange Commission on December 14, 2018, by: (i) Adriatic Acquisition Corporation, a Delaware corporation (“Purchaser”) and a direct wholly-owned subsidiary of GlaxoSmithKline LLC, a limited liability company organized under the laws of Delaware (“GSK LLC”), which is an indirect wholly-owned subsidiary of GlaxoSmithKline plc, a public limited company organized under the laws of England and Wales (“Parent”), (ii) GSK LLC and (iii) Parent. The Schedule TO relates to the offer by Purchaser to purchase all of the issued and outstanding shares of common stock, par value $0.0001 per share (each, a “Share” and collectively, the “Shares”) of TESARO, Inc., a Delaware corporation (the “Company”), for $75.00 per Share, net to the holder in cash, without interest, subject to any withholding taxes required by applicable law, and on the terms and subject to the conditions set forth in the Offer to Purchase, dated December 14, 2018 (together with any amendments or supplements thereto, the “Offer to Purchase”) and in the accompanying Letter of Transmittal (together with any amendments or supplements thereto, the “Letter of Transmittal,” and together with the Offer to Purchase, the “Offer”), copies of which are attached to the Schedule TO as Exhibits (a)(1)(a) and (a)(1)(b), respectively. The information set forth in the Offer to Purchase, including all schedules thereto, and the related Letter of Transmittal is incorporated herein by reference with respect to all of the applicable items in the Schedule TO, except that such information is hereby amended and supplemented to the extent provided herein.

This Amendment is being filed on behalf of Parent, GSK LLC and Purchaser. All capitalized terms used in this Amendment and not otherwise defined have the respective meanings ascribed to them in the Schedule TO. Except as otherwise set forth in this Amendment, the information set forth in the Schedule TO remains unchanged and is incorporated herein by reference to the extent relevant to the items in this Amendment. This Amendment should be read together with the Schedule TO.

The Offer to Purchase and Item 11 of the Schedule TO, to the extent Item 11 incorporates by reference the information contained in the Offer to Purchase, are hereby amended and supplemented as set forth below:

The information set forth in Section 16. “Certain Legal Matters; Regulatory Approvals” is hereby amended and supplemented by adding the following paragraphs as new paragraphs after the second paragraph under the heading “Foreign Approvals”:

Parent filed a notification with the FCO in connection with the Offer on December 18, 2018. The one (1)-month review period is scheduled to expire at 11:59 P.M., Central European Time, on January 18, 2019, unless (i) the FCO issues a clearance decision prior to the expiration of the review period or (ii) the FCO notifies Parent within the review period after the date of filing of the initiation of an in-depth investigation.

Parent filed a notification with the FCA in connection with the Offer on December 18, 2018. The four (4) week review period is scheduled to expire at 11:59 P.M., Central European Time on January 15, 2019, unless (i) the FCA issues a waiver prior to the expiration of the review period or (ii) a motion is filed with the Austrian Cartel Court to review the transactions.

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SIGNATURE

After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: December 18, 2018

EXHIBIT INDEX