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 May 22,  1998 the closing  price of the
Common  Stock as reported on the New York Stock  Exchange was $47.625 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  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 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  owner of the Common
Stock.

                                        Amount of Shares           Amount of
                                       Beneficially Owned        Shares to be
Name of Selling Shareholder(s)      as of March 22, 1998(1)    Offered for Sale
- ------------------------------      -----------------------    ----------------
3423611 Canada Inc.(1)                       120,358              120,358

(1)   The ultimate controlling shareholder is an officer and director of GPC and
      a subsidiary of GPC.

      Except for the  purchase  of the  shares of Common  Stock,  their  current
holdings of Exchangeable  Shares 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.

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

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 May 26, 1998.