PROSPECTUS SUPPLEMENT
(To Prospectus Dated January 28, 1998)

                         807,746 Shares of Common Stock
                                ($.15 Par Value)

                               OMNICOM GROUP INC.

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      This document  supplements the Prospectus  dated January 28, 1998 relating
to 807,746 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 GPC International  Holdings Inc.
("GPC"),  a  corporation  incorporated  under  the laws of  Canada,  issued in a
private  offering in Canada by GPC 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 September 12, 2000, the closing price of
the  Common  Stock as  reported  on the New York Stock  Exchange  was $82.38 per
share. The Common Stock is traded under the symbol "OMC."

      The Selling  Shareholder do not own in excess of 1% of the Common Stock of
the Company and since the Selling  Shareholder 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 the 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
    Names of Selling                 Beneficially Owned           Shares to be
    Shareholder(s)                as of September 12, 2000      Offered for Sale
    ----------------              ------------------------      ----------------
    1067913 Ontario Limited(1)             15,060                    15,060

    (1) James  Crossland,  the  controlling  shareholder,  is an officer  and/or
        director of GPC and a subsidiary of GPC and  indirectly  owns 290 shares
        of Common.

      Except for the purchase of the shares of Common Stock and the  information
disclosed in the footnotes above, 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 September 18, 2000.