PROSPECTUS SUPPLEMENT
(To Prospectus Dated January 28, 1998)


                               OMNICOM GROUP INC.
                         219,205 Shares of Common Stock
                                ($.15 Par Value)

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      This document supplements the Prospectus dated January 28, 1998 relating
to 219,205 shares of Common Stock, par value $.15 per share (the "Common Stock")
of the Omnicom Group Inc., a New York corporation ("Omnicom" or the "Company").
Each share of Common Stock offered hereby was issued upon exchange of an
Exchangeable Share (an "Exchangeable Share") of Omnicom Canada Inc. ("OCI"), a
corporation incorporated under the laws of Ontario, originally issued in a
private offering in Canada by Palmer Jarvis Inc. (now amalgamated into OCI) and
the Company. The shares of Common Stock offered hereby were initially acquired
upon exchange of the Exchangeable Shares in a private offering in Canada. This
Prospectus Supplement is incorporated by reference into the Prospectus, and all
terms used herein shall have the meaning assigned to them in the Prospectus. On
March 19, 2001 the closing price of the Common Stock as reported on the New York
Stock Exchange was $82.31 per share. The Common Stock is traded under the symbol
"OMC."

      None of the Selling Shareholders owns in excess of 1% of the Common Stock
of the Company and since the Selling Shareholders may sell all, some or none of
the shares of Common Stock offered hereby, no estimate can be made of the
aggregate number of shares of Common Stock that will be owned by each Selling
Shareholder upon completion of the offering to which this Prospectus Supplement
relates. In accordance with the Section of the Prospectus entitled "Selling
Shareholders" (which appears on page 10 of the Prospectus), the following
information is provided with respect to the beneficial owners of the Common
Stock:

                                         Amount of Shares          Amount of
                                        Beneficially Owned       Shares to be
Name of Selling Shareholder(s)         as of March 19, 2001    Offered for Sale
------------------------------        ----------------------   ----------------
Roald Glyn Thomas                            12,000(1)               2,000

(1)   Includes 10,000 shares of Common Stock which Roald Glyn Thomas has the
      right to acquire upon the exchange of 10,000 Exchangeable Shares.

      Except for the purchase of the shares of Common Stock and his position
with OCI, the Selling Shareholder has not had a material relationship with the
Company or any of its affiliates within the past three years.

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      THESE  SECURITIES  HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES
AND EXCHANGE  COMMISSION OR ANY STATE SECURITY COMMISSION NOR HAS THE SECURITIES
AND  EXCHANGE  COMMISSION  OR ANY STATE  SECURITIES  COMMISSION  PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.

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           The date of this Prospectus Supplement is March 23, 2001.