<PAGE>
                                                                     Exhibit 4.2
 
                         AMENDMENT TO RIGHTS AGREEMENT
                         -----------------------------
                                        
          Amendment, dated as of August 8, 1998, to the Rights Agreement (as
amended, the "Rights Agreement"), dated as of September 13, 1996, between
Opinion Research Corporation, a Delaware corporation (the "Company"), and
StockTrans, Inc., a Pennsylvania corporation, as rights agent (the "Rights
Agent").

          Pursuant to Section 26 of the Rights Agreement, the Company and the
Rights Agent may from time to time supplement or amend any provision of the
Rights Agreement in accordance with such Section 26.  All acts and things
necessary to make this Amendment a valid agreement, enforceable according to its
terms, have been done and performed, and the execution and delivery of this
Amendment by the Company and the Rights Agent have been in all respects duly
authorized by the Company and the Rights Agent.

          In consideration of the foregoing and the mutual agreements set forth
herein, the parties hereto have agreed as follows:

          1.   The Rights Agreement is hereby amended by deleting Section 1(g)
thereof in its entirety.

          2.   Section 23(a) of the Rights Agreement is hereby amended by
deleting the words "there must be Continuing Directors then in office and such
authorization shall require the concurrence of a majority of such Continuing
Directors" and substituting therefor the words "such authorization shall require
the concurrence of 75% of the members of the Board of Directors of the Company".

          3.   Section 26 of the Rights Agreement is hereby amended:

                    (a) by deleting the parenthetical "(which lengthening or
               shortening, following the first occurrence of an event set forth
               in clauses (i) and (ii) of the first proviso to Section 23(a)
               hereof, shall be effective only if there are Continuing Directors
               and shall require the concurrence of a majority of such
               Continuing Directors)" in its entirety; and

                    (b) by inserting the following words at the end of the
               penultimate sentence of Section 26: "and following the first
               occurrence of an event set forth in clauses (i) and (ii) of the
               first proviso to Section 23(a) hereof, any supplement or
               amendment shall require the concurrence of 75% of the members of
               the Board of Directors of the Company".

          4.   Section 28 of the Rights Agreement is hereby amended:
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                    (a) by deleting the words "the Continuing Directors" the
               first three times such words appear and substituting therefor the
               words "75% of the members of the Board of Directors of the
               Company"; and

                    (b) by deleting the words "or the Continuing Directors" in
               clause (y) of the last sentence of such Section 28.

          5.   Section 30 of the Rights Agreement is hereby amended by deleting
the last sentence thereof in its entirety.

          6.   Exhibit B to the Rights Agreement is hereby amended by deleting
the words "a majority of the Continuing Directors (as such term is defined in
the Rights Agreement)" and substituting therefor "75% of the members of the
Board of Directors of the Company".

          7.   Exhibit C to the Rights Agreement is hereby amended:

                    (a) by deleting the words "a majority of the Continuing
               Directors (as defined below)"  in the tenth paragraph thereof and
               substituting therefor the words "75% of the members of the Board
               of Directors of the Company";

                    (b) by deleting the eleventh paragraph thereof which begins
               "The term `Continuing Director' means" in its entirety;

                    (c) by adding the words "(in certain circumstances, with the
               concurrence of 75% of the members of the Board of Directors of
               the Company)" to the end of the first sentence of the thirteenth
               paragraph thereof; and

                    (d) by deleting the words "the Continuing Directors" in the
               thirteenth paragraph thereof and substituting therefor the words
               "75% of the members of the Board of Directors of the Company".
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          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
the Rights Agreement to be duly executed and attested, all as of the date and
year first above written.

Attest:                                 OPINION RESEARCH CORPORATION



By: /s/ James T. Heisler                By: /s/ John F. Short
    ----------------------------            ---------------------
  Name:  James T. Heisler               Name:  John F. Short
  Title: Director and                   Title:   President
         Executive Vice President


Attest:                                 STOCKTRANS, INC.



By: /s/ Lisa Ann Klevence               By: /s/ Jonathan Miller
    -----------------------------           ---------------------
  Name: Lisa Ann Klevence               Name: Jonathan Miller
  Title:  Vice President, Operations    Title:  President