PROSPECTUS SUPPLEMENT
(To Prospectus Dated February 26, 2001)
                                                Filed Pursuant to Rule 424(b)(3)
                                            Registration Statement No. 333-55386

                                  $850,000,000

                               OMNICOM GROUP INC.
                      Liquid Yield Option(TM) Notes due 2031
                              (Zero Coupon-Senior)

      This  prospectus   supplement   relates  to  the  resale  by  the  selling
securityholders  of Liquid  Yield  Option(TM)  Notes (Zero Coupon -- Senior) due
2031 (the "LYONs") of Omnicom Group Inc. and the shares of common stock issuable
upon the conversion and/or redemption of the LYONs.

      This  prospectus  supplement  should  be  read  in  conjunction  with  the
prospectus  dated  February  26,  2001,  which  is to  be  delivered  with  this
prospectus  supplement.  All  capitalized  terms  used  but not  defined  in the
prospectus supplement shall have the meanings given them in the prospectus.

      The  following  information  replaces  in  its  entirety  the  information
provided in the prospectus under the caption "Selling Securityholders."

                             SELLING SECURITYHOLDERS

      The LYONs were originally issued by us and sold by Merrill Lynch,  Pierce,
Fenner & Smith  Incorporated  in a  transaction  exempt  from  the  registration
requirements  of the  Securities Act to persons  reasonably  believed by Merrill
Lynch to be "qualified  institutional buyers" (as defined by Rule 144A under the
Securities  Act).  The  selling   securityholders  (which  term  includes  their
transferees,  pledgees,  donees or  successors)  may from time to time offer and
sell pursuant to this prospectus, as amended or supplemented, any and all of the
LYONs and the shares of common stock issuable upon conversion  and/or redemption
of the LYONs.

      Set  forth  below  are the  names  of  each  selling  securityholder,  the
principal  amount of LYONs that may be offered  by such  selling  securityholder
pursuant  to this  prospectus,  as  amended or  supplemented,  and the number of
shares of common stock into which such LYONs are  convertible.  Unless set forth
below, none of the selling  securityholders has had a material relationship with
us or any of our predecessors or affiliates within the past three years.

      The following  table sets forth certain  information  received by us on or
prior to March 15, 2001. However, any or all of the LYONs or common stock listed
below may be  offered  for sale  pursuant  to this  prospectus,  as  amended  or
supplemented,  by the selling securityholders from time to time. Accordingly, no
estimate  can be given as to the  amounts of LYONs or common  stock that will be
held by the selling  securityholders  upon  consummation  of any such sales.  In
addition,   the  selling   securityholders   identified  below  may  have  sold,
transferred  or otherwise  disposed of all or a portion of their LYONs since the
date  on  which  the  information   regarding  their  LYONs  was  provided,   in
transactions exempt from the registration requirements of the Securities Act.



                                                                                              Common Stock
                                                     Aggregate Principal Amount  Percentage   Beneficially   Common Stock
                                                        of LYONs at Maturity      of LYONs    Owned Prior     Registered
                        Name                              that may be Sold       Outstanding  to Conversion    Hereby(1)
-------------------------------------------------    --------------------------  -----------  -------------   -----------
                                                                                                 
AAM/Zazove Institutional Income Fund L.P. (BS) ......       $  1,000,000                *            --           9,090
AIG SoundShore Holdings Ltd. ........................          6,451,000                *            --          58,639
AIG SoundShore Opportunity Holding Fund Ltd. ........          7,328,000                *            --          66,611
AIG SoundShore Strategic Holding Fund Ltd. ..........          4,221,000                *            --          38,368
Allstate Insurance Company ..........................          1,700,000                *        85,300          15,453
Allstate Life Insurance Company .....................          2,300,000                *        85,300          20,907
Arpeggio Fund, L.P. .................................          1,000,000                *            --           9,090
Bear, Stearns & Co., Inc. ...........................          2,500,000                *            --          22,725
First Union National Bank ...........................         21,000,000             2.47%           --         190,890
GLG Global Convertible Fund .........................          1,650,000                *            --          14,998
GLG Global Convertible Ucits Fund ...................            350,000                *            --           3,181
GLG Market Neutral Fund .............................          2,500,000                *            --          22,725
HFR Master Fund LTD .................................            200,000                *            --           1,818
HighBridge International LLC ........................         30,000,000             3.53%           --         272,700
IMF Convertible Fund ................................            700,000                *            --           6,363
Investcorp - SAM Fund Ltd. ..........................          3,200,000                *            --          29,088
KBC Financial Products USA ..........................          1,000,000                *            --           9,090
Merrill Lynch, Pierce, Fenner & Smith Incorporated(2)         53,350,000             6.28%           --         484,951
Morgan Stanley & Co. Incorporated(3) ................         15,000,000             1.76%           --         136,350
Nomura Securities International, Inc. ...............         15,000,000             1.76%      307,158         136,350
Onyx Fund Holdings, LDC .............................         12,000,000             1.41%           --         109,080
R2 Investments, LDC .................................        122,000,000            14.35%           --       1,108,980
Rhapsody Fund, LP ...................................          3,700,000                *            --          33,633
Royal Bank of Canada ................................          6,000,000                *        95,033          54,540
Sagamore Hill Hub Fund Ltd. .......................           14,500,000             1.71%           --         131,805
Salomon Smith Barney Inc. ...........................          3,000,000                *            --          27,270
SAM Investments LDC .................................         50,000,000             5.88%           --         454,500
San Diego County Employees Retirement Association ...          2,800,000                *            --          25,452
St. Albans Partners Ltd. ............................         13,000,000             1.53%           --         118,170
Tribeca Investments LLC .............................         25,000,000             2.94%           --         227,250
White River Securities L.L.C. .......................          2,500,000                *            --          22,725
All other holders of LYONs or future transferees,
pledgees, donees or successors of any such
holders(4)(5) .......................................        425,050,000            56.38%           --       3,863,708
                                                            ------------           ------       -------      ----------
         Total ......................................       $850,000,000           100.00%      572,791       7,726,500
                                                            ============           ======       =======      ==========

*  Less than 1%

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(1)   Assumes  conversion of all of the holder's  LYONs at a conversion  rate of
      9.09 shares of common stock per $1,000 principal amount at maturity of the
      LYONs.  However,  this  conversion  rate will be subject to  adjustment as
      described  under  "Description  of the LYONs --  Conversion  Rights." As a
      result,  the amount of common stock issuable upon  conversion of the LYONs
      may increase or decrease in the future.

(2)   Merrill  Lynch,  Pierce,  Fenner  & Smith  Incorporated  was  the  initial
      purchaser in the private  placement on February 7, 2001 in which the LYONs
      were originally issued.  Merrill Lynch has advised us that it is not aware
      of any position,  office or directorship relationship that it has had with
      Omnicom or its affiliates.  However,  Merrill Lynch has advised us that it
      may have,  from time to time,  acted in a  financial  investment  advisory
      capacity to Omnicom.

(3)   Morgan  Stanley  & Co.  Incorporated  was  the  initial  purchaser  in the
      offering of $230,000,000  aggregate principal amount of 2 1/4% convertible
      subordinated debentures due 2013 on January 6, 1998 by Omnicom.

(4)   Information  about  other  selling  securityholders  will be set  forth in
      prospectus supplements, if required.

(5)   Assumes  that any other  holders  of  LYONs,  or any  future  transferees,
      pledgees, donees or successors of or from any such other holders of LYONs,
      do not  beneficially  own any common  stock  other  than the common  stock
      issuable upon conversion of the LYONs at the initial conversion rate.

      The preceding table has been prepared based upon information  furnished to
us by the  selling  securityholders  named  in the  table.  From  time to  time,
additional  information  concerning  ownership of the LYONs and common stock may
rest with certain holders thereof not named in the preceding table, with whom we
believe we have no affiliation.  Information  about the selling  securityholders
may  change  from  over  time.  Any  changed  information  will be set  forth in
prospectus supplements.
                                   ----------
           The date of this Prospectus Supplement is March 16, 2001.

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