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                                                                     Exhibit 5.1
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                [LETTERHEAD OF HALE AND DORR LLP APPEARS HERE]


                               September 2, 1999


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

     Re:  Registration Statement on Form S-4
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Ladies and Gentlemen:

     This opinion is furnished to you in connection with a Registration
Statement on Form S-4 (the "Registration Statement") filed with the Securities
and Exchange Commission (the "Commission") under the Securities Act of 1933, as
amended (the "Securities Act"), for the registration of up to 7,700,000 shares
of Common Stock, $.001 par value per share (the "Shares"), of Open Market, Inc.,
a Delaware corporation (the "Company"), issuable pursuant to the Agreement and
Plan of Merger, dated as of July 14, 1999, by and among the Company, OM/SA
Acquisition Corporation and FutureTense, Inc. (the "Merger Agreement").

     We are acting as counsel for the Company in connection with the issuance by
the Company of the Shares. We have examined signed copies of the Registration
Statement as filed with the Commission. We have also examined and relied upon
the Merger Agreement, minutes of meetings of the Board of Directors of the
Company as provided to us by the Company, record books of the Company as
provided to us by the Company, the Certificate of Incorporation and Bylaws of
the Company, each as restated and/or amended to date, and such other documents
as we have deemed necessary for purposes of rendering the opinions hereinafter
set forth.

     In our examination of the foregoing documents, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as originals, the conformity to original documents of all documents submitted to
us as copies, the authenticity of the originals of such latter documents and the
legal competence of all signatories to such documents.

     We assume that the appropriate action will be taken, prior to the issuance
of the Shares in accordance with the Merger Agreement, to register and qualify
the Shares for sale under all applicable state securities or "blue sky" laws.
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Open Market, Inc.
September 2, 1999
Page 2

     We express no opinion herein as to the laws of any state or jurisdiction
other that the state laws of the Commonwealth of Massachusetts, the Delaware
General Corporation Law statute and the federal laws of the United States of
America. To the extent that any other laws govern the matters as to which we are
opining herein, we have assumed that such laws are identical to the state laws
of the Commonwealth of Massachusetts, and we are expressing no opinion herein as
to whether such assumption is reasonable or correct.

     Based upon and subject to the foregoing, we are of the opinion that the
Shares to be issued by the Company have been duly authorized for issuance and,
when such Shares are issued in accordance with the terms and conditions of the
Merger Agreement, such Shares will be validly issued, fully paid and
nonassessable.

     It is understood that this opinion is to be used only in connection with
the issuance of the Shares while the Registration Statement is in effect.

     Please note that we are opining only as to the matters expressly set forth
herein, and no opinion should be inferred as to any other matters. This opinion
is based upon currently existing statutes, rules, regulations and judicial
decisions, and we disclaim any obligation to advise you of any change in any of
these sources of law or subsequent legal or factual developments which might
affect any matters or opinions set forth herein.

     We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act and to the use of our
name therein and in the related Prospectus under the caption "Legal Matters." In
giving such consent, we do not hereby admit that we are in the category of
persons whose consent is required under Section 7 of the Securities Act or the
rules and regulations of the Commission.

                                         Very truly yours,

                                         /s/ Hale and Dorr LLP
                                         HALE AND DORR LLP