PROSPECTUS SUPPLEMENT
(To Prospectus Dated January 28, 1998)

                         219,205 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 219,205 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 Palmer Jarvis Inc. ("PJI"),  a
corporation incorporated under the laws of British Columbia, issued in a private
offering in Canada by PJI 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 June 5, 1998 the  closing  price of the
Common  Stock as reported on the New York Stock  Exchange was $46.625 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 June 5, 1998(1)       Offered for Sale
- ------------------------------     ----------------------       ----------------
Roald Glyn Thomas                          23,467                    9,000

(1)   Includes  14,467  shares of Common  Stock  which Roald Glyn Thomas has the
      right to acquire upon the exchange of 14,467 Exchangeable Shares.

      Except for the  purchase  of the  shares of Common  Stock,  their  current
holdings  of  Exchangeable  Shares  and their  positions  with PLJ,  none of the
Selling Shareholders 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 June 9, 1998.