PROSPECTUS SUPPLEMENT
(To Prospectus Dated January 28, 1998)

                         807,746 Shares of Common Stock
                                ($.50 Par Value)

                               OMNICOM GROUP INC.
                              ---------------------

     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 April 20, 1998 the closing price of the
Common Stock as reported on the New York Stock  Exchange was $47.3125 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 April 20, 1998(1)    Offered for Sale
- ------------------------------      -----------------------    ----------------
Societal Ventures Corporation(2)             98,908                18,908
3119025 Canada Inc.(3)(4)                    45,024                35,000
3423611 Canada Inc.(3)                       420,358              300,000
1067913 Ontario Limited(3)(5)                70,060                17,500

(1)  Includes  (a)  80,000  shares  of  Common  Stock  which  Societal  Ventures
     Corporation   has  the  right  to  acquire  upon  the  exchange  of  80,000
     Exchangeable Shares, (b) 10,024 shares of Common Stock which 3119025 Canada
     Inc.  has the right to acquire  upon the  exchange  of 10,024  Exchangeable
     Shares,  (c) 120,358  shares of Common Stock which 3423611  Canada Inc. has
     the right to acquire upon the exchange of 120,358  Exchangeaable Shares and
     (d) 52,560  shares of Common Stock which  1067913  Ontario  Limited has the
     right to acquire upon the exchange of 52,560  Exchangeable  Shares.  
(2)  Societal Ventures  Corporation  renders consulting services to a subsidiary
     of GPC.
(3)  The  controlling  shareholder  is an officer  and/or  director of GPC and a
     subsidiary of GPC.
(4)  Patrick Ross,  controlling  shareholder and director,  directly owns 25,036
     Exchangeable Shares.
(5)  James Crossland,  controlling shareholder and director, indirectly owns 290
     shares of Common Stock.

     Except  for the  purchase  of the  shares of Common  Stock,  their  current
holdings of Exchangeable  Shares and the information  disclosed in the footnotes
above, none of the Selling Shareholders has had a material relationship with the
Company or any of its affiliates within the past three years.

                             -----------------------

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 April 22, 1998.