<PAGE>

                               Hale and Dorr LLP


                             Counsellors  at  Law

                 60 State Street, Boston, Massachusetts  02109
                        617-526-6000  fax 617-526-5000


                                                                     Exhibit 8.1
                                                                     -----------

                              ______, 1999



Open Market, Inc.
1 Wayside Road
Burlington, MA  01803

     Re:  Merger pursuant to Agreement and Plan of Merger among
          Open Market, Inc., OM/SA Acquisition and FutureTense, Inc.
          ----------------------------------------------------------

Ladies and Gentlemen:

     This opinion is being delivered to you in connection with the filing of a
registration statement (the "Registration Statement") on Form S-4, which
includes the Joint Proxy Statement and Prospectus relating to the Agreement and
Plan of Merger dated as of July 14, 1999 (the "Merger Agreement"), by and among
Open Market, Inc., a Delaware corporation ("Parent"), OM/SA Acquisition
Corporation, a Delaware corporation and wholly owned subsidiary of Parent
("Sub"), and FutureTense, Inc., a Delaware corporation ("Target").  Pursuant to
the Merger Agreement, Sub will merge with and into Target (the "Merger").
Except as otherwise provided, capitalized terms not defined herein have the
meanings set forth in the Merger Agreement and the exhibits thereto or in the
letters delivered to Hale and Dorr LLP by Parent and Target containing certain
representations of Parent and Target relevant to this opinion (the
"Representation Letters").  All section references, unless otherwise indicated,
are to the United States Internal Revenue Code of 1986, as amended (the "Code").

     In our capacity as counsel to Parent in the Merger, and for purposes of
rendering this opinion, we have examined and relied upon the Registration
Statement, the Merger Agreement and the exhibits thereto, the Representation
Letters, and such other documents as we considered relevant to our analysis.  In
our examination of documents, we have assumed the authenticity of original
documents, the accuracy of copies, the genuineness of signatures, and the legal
capacity of signatories.

     We have assumed that all parties to the Merger Agreement and to any other
documents examined by us have acted, and will act, in accordance with the terms
of such Merger Agreement and documents and that the Merger will be consummated
at
<PAGE>

Open Market, Inc.
              , 1999
-------------
Page 2


the Effective Time pursuant to the terms and conditions set forth in the
Merger Agreement without the waiver or modification of any such terms and
conditions.  Furthermore, we have assumed that all representations contained in
the Merger Agreement, as well as those representations contained in the
Representation Letters, are, and at the Effective Time will be, true and
complete in all material respects, and that any representation made in any of
the documents referred to herein "to the best of the knowledge and belief" (or
similar qualification) of any person or party is correct without such
qualification.  We have also assumed that as to all matters for which a person
or entity has represented that such person or entity is not a party to, does not
have, or is not aware of, any plan, intention, understanding, or agreement,
there is no such plan, intention, understanding, or agreement.  We have not
attempted to verify independently such representations, but in the course of our
representation, nothing has come to our attention that would cause us to
question the accuracy thereof.

     The conclusions expressed herein represent our judgment as to the proper
treatment of certain aspects of the Merger under the income tax laws of the
United States based upon the Code, Treasury Regulations, case law, and rulings
and other pronouncements of the Internal Revenue Service (the "IRS") as in
effect on the date of this opinion.  No assurances can be given that such laws
will not be amended or otherwise changed prior to the Effective Time, or at any
other time, or that such changes will not affect the conclusions expressed
herein.  Nevertheless, we undertake no responsibility to advise you of any
developments after the Effective Time in the application or interpretation of
the income tax laws of the United States.

     Our opinion represents our best judgment of how a court would decide if
presented with the issues addressed herein and is not binding upon either the
IRS or any court.  Thus, no assurances can be given that a position taken in
reliance on our opinion will not be challenged by the IRS or rejected by a
court.

     This opinion addresses only the specific United States federal income tax
consequences of the Merger set forth below, and does not address any other
federal, state, local, or foreign income, estate, gift, transfer, sales, use, or
other tax consequences that may result from the Merger or any other transaction
(including any transaction undertaken in connection with the Merger).  We
express no opinion regarding the tax consequences of the Merger to shareholders
of Target that are subject to special tax rules, and we express no opinion
regarding the tax consequences of the Merger arising in connection with the
ownership of options or warrants for Target stock.

     On the basis of, and subject to, the foregoing, and in reliance upon the
representations and assumptions described above, we are of the following
opinions:

     1.   The Merger will constitute a reorganization within the meaning of
Section 368(a) and
<PAGE>

Open Market, Inc.
              , 1999
-------------
Page 3


     2.  The discussion under the section "Material United States Federal
Income Tax Consequences" in the Registration Statement accurately describes the
material federal income tax considerations relevant to Target stockholders
receiving Parent Common Stock in the Merger.

     In rendering this opinion, we have assumed that Testa, Hurwitz & Thibeault,
LLP has delivered, and has not withdrawn, an opinion that is substantially
similar to this one.  No opinion is expressed as to any federal income tax
consequence of the Merger except as specifically set forth herein, and this
opinion may not be relied upon except with respect to the consequences
specifically discussed herein.

     This opinion is intended solely for the purpose of inclusion as an exhibit
to the Registration Statement.  It may not be relied upon for any other purpose
or by any other person or entity, and may not be made available to any other
person or entity without our prior written consent.  We hereby consent to the
filing of this opinion as an exhibit to the Registration Statement and further
consent to the use of our name in the Registration Statement in connection with
references to this opinion and the tax consequences of the Merger.  In giving
this consent, however, we do not hereby admit that we are in the category of
persons whose consent is required under Section 7 of the Securities Act of 1933,
as amended.

                              Very truly yours,



                              Hale and Dorr LLP