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                                                                    EXHIBIT 5.1

                      [COHEN & GRIGSBY, P.C. LETTERHEAD]



                               February 28, 1997


Board of Directors of
F.N.B. Corporation
Hermitage Square
Hermitage, PA 16148

Gentlemen:

         We have been asked as to render this opinion in connection with the
filing by F.N.B. Corporation, a Pennsylvania corporation (the "Company"), of a
Registration Statement on Form S-4 (the "Registration Statement") with the
Securities and Exchange Commission to register shares of the Company's common
stock, par value $2.00 per share (the "Common Stock"), to be issued to the
shareholders of West Coast Bancorp, Inc., a Florida corporation ("West Coast"),
upon consummation of the merger of West Coast with and into Southwest Banks,
Inc., a Florida corporation ("Southwest") (the "Merger") in accordance with the
terms of the Agreement and Plan of Merger (the "Agreement") dated as of
November 15, 1996 among the Company, Southwest and West Coast.

         We have not represented the Company in connection with the
negotiation, execution or delivery of the Agreement or the Merger. In rendering
this Opinion, we have made no investigation or inquiry other than review of the
Agreement, the draft Registration Statement and the resolutions adopted by the
Executive Committee of the Board of Directors of the Company on September 23,
1996 with respect to the Merger, the Agreement and the transactions related
thereto.

         In our opinion the shares of Common Stock to be issued to the
shareholders of West Coast upon consummation of the Merger, when issued in
accordance with the terms of the Agreement, will be validly issued, fully-paid
and non-assessable shares of Common Stock of the Company.

         We hereby consent to the reference to us in the Prospectus/Proxy
Statement of the Company and West Coast constituting part of the Registration



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COHEN & GRIGSBY, P.C.

      Boards of Directors of
      F.N.B. Corporation
      February 28, 1997
      Page 2



Statement and to the inclusion of this letter as an exhibit to the Registration
Statement.

         This Opinion Letter is governed by, and shall be interpreted in
accordance with, the Legal Opinion Accord (the "Accord") of the ABA Section of
Business Law (1991) as supplemented or modified by Part I, together with the
Forward and Glossary of the Pennsylvania Third Party Legal Opinion Supplement
(the "Pennsylvania Supplement") of the PBA Section of Corporation, Banking and
Business Law (1992). As a consequence, it is subject to a number of
qualifications, exceptions, definitions, limitations on coverage and other
limitations, all as more particularly described in the Accord and the
Pennsylvania Supplement, and this Opinion Letter should be read in conjunction
therewith. Unless otherwise indicated, capitalized terms used in this Opinion
that are defined in the Accord or the Pennsylvania Supplement will have the
same meanings in this Opinion as the meanings set forth in the Accord or the
Pennsylvania Supplement, respectively (and, to the extent of a conflict between
the same, priority shall be given to the Accord and the Pennsylvania Supplement
in that order).

         This Opinion Letter may be relied upon by you only in connection with
the transactions contemplated by the Agreement and may not be used or relied
upon by you or any other person for any purpose whatsoever, except to the
extent authorized in the Accord, without in each instance our prior written
consent.


                                      Very truly yours,



                                       /s/ Cohen & Grigsby, P.C.
                                       -------------------------
                                       COHEN & GRIGSBY, P.C.



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