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                                                                       EXHIBIT 5

                             COHEN & GRIGSBY, P.C.
                                ATTORNEYS AT LAW
                         11 STANWIX STREET, 15TH FLOOR
                      PITTSBURGH, PENNSYLVANIA 15222-3115

                            TELEPHONE (412) 297-4900
                               FAX (412) 209-0672

                                  May 31, 2000

F.N.B. Corporation
One F.N.B. Boulevard
Hermitage, PA  16148

Gentlemen:

    We have acted as counsel to F.N.B. Corporation, a Pennsylvania corporation
(the "Company"), in connection with the preparation and filing with the
Securities and Exchange Commission of a Registration Statement on Form S-3 (the
"Registration Statement") in order to register under the Securities Act of 1933,
as amended, $250,000,000 aggregate principal amount of (i) Subordinated Term
Notes due 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 36, 48, 60, 84 and 120 Months (in
the aggregate, the "Term Notes"), (ii) Subordinated Daily Notes (the "Daily
Notes") and (iii) Special Subordinated Daily Notes (the "Special Daily Notes")
of the Company (the Term Notes, the Daily Notes and the Special Daily Notes are
collectively referred to herein as the "Securities") to be issued under an
Indenture, dated as of May 15, 1992, between the Company and Chase Manhattan
Trust Company, National Association, successor to Northern Central Bank, as
Trustee (the "Trustee"), as supplemented by the First Supplemental Indenture
between the Company and the Trustee dated as of January 1, 1994 (collectively
the "Indenture").

    In connection with the foregoing, we have examined:

    (a) The Registration Statement and the exhibits and amendments thereto;

    (b) The Company's Articles of Incorporation, as amended, and Bylaws, as
        amended;

    (c) The Indenture;

    (d) The form of Officer's Certificate of the Company setting forth the terms
        of the Term Notes, the Daily Notes and the Special Daily Notes;

    (e) Such other corporate records and documents as we have considered
        relevant, necessary or appropriate for purposes of this opinion.

    We have also assumed:

        (i) the due authentication by or on behalf of the Trustee and the due
    execution and delivery by the Company of the Securities; and

        (ii) the issuance and sale of the Securities under the Indenture as
    described in the Registration Statement, including receipt by the Company of
    the full consideration for the Securities set forth therein.

    Based upon such examination and assumptions, we are of the opinion that the
Securities and the Indenture will be enforceable obligations of the Company.


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    We hereby consent to the use of our name in the Registration Statement under
the caption "Legal Matters" in the Prospectus which constitutes part of the
Registration Statement and to the filing of this opinion 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).

                                                           Very truly yours,

                                                           COHEN & GRIGSBY, P.C.

CJR:JDP/eph