PROSPECTUS SUPPLEMENT
(To Prospectus Dated January 28, 1998)

                         807,746 Shares of Common Stock
                                ($.50 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 $.50 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 August 3, 1998 the closing price of the
Common  Stock as reported on the New York Stock  Exchange  was $51.25 per share.
The Common Stock is traded under the symbol "OMC".

      The Selling  Shareholder  does 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
                                       Beneficially Owned         Shares to be
Name of Selling Shareholder(s)      as of August 3, 1998(1)     Offered for Sale
- --------------------------------    ----------------------      ----------------
Societal Ventures Corporation(2)             60,000                  32,100

(1)   Includes 27,900 shares of Common Stock which Societal Ventures Corporation
      has the right to acquire upon the exchange of 27,900 Exchangeable Shares.

(2)   Societal Ventures  Corporation renders consulting services to a subsidiary
      of GPC.

      Except for the purchase of the shares of Common Stock and the  information
disclosed in the footnote 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.
                             -----------------------

            The date of this Prospectus Supplement is August 7, 1998.