
<PAGE>
 
                                                                     EXHIBIT 5.1

                     [LETTERHEAD OF O'MELVENY & MYERS LLP]

                                    May
                                    7th
                                    1 9 9 8

(310) 553-6700
                                                                       043,280-1


Balance Bar Company
1015 Mark Avenue
Carpinteria, CA 93013

Dear Ladies and Gentlemen:

          Balance Bar Company (the "Company") is registering up to 2,760,000
shares of its Common Stock, par value $0.01 per share, of which 1,003,372 shares
are being sold by the Company (the "Company Shares") and up to 1,756,628 shares
are being sold by certain selling stockholders of the Company (the "Stockholder
Shares," and together with the Company Shares, the "Shares"), under the
Securities Act of 1933, as amended (the "Act"), pursuant to a Registration
Statement on Form S-1 (the "Registration Statement"), filed with the Securities
and Exchange Commission on April 8, 1998.  In connection with this transaction,
you have requested our opinion with respect to the matters set forth below.

          We have examined such matters of fact and questions of law as we have
considered appropriate for purposes of rendering the opinion expressed below.

          For the purposes of this opinion, we are assuming that (a) the Amended
and Restated Certificate of Incorporation (the "Certificate of Incorporation"),
effecting a six-for-one stock split that entitles each holder of capital stock
of the Company to receive five additional shares of such capital stock for each
share held (the "Stock Split"), will be filed with the Delaware Secretary of
State before the issuance of shares in connection with the Stock Split (the
"Split Shares"), (b) the conversion of all outstanding shares of the Company's
preferred stock, which is conditioned upon the declaration of the effectiveness
of the Registration Statement, occurs, and (c) the shares of Common Stock 
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Page 2 - Balance Bar Company - May 7, 1998

that are to be issued upon conversion of all outstanding shares of the Company's
preferred stock (the "Conversion Shares," and together with the Split Shares and
Company Shares, the "Newly Issued Shares") will be issued in conformity with the
terms of the Company's Restated Certificate of Incorporation.

          We are opining herein as to the effect on the subject transaction of
only the General Corporation Law of the State of Delaware and we express no
opinion with respect to the applicability thereto or the effect thereon of any
other laws or as to any matters of municipal law or any other local agencies
within any state.

          Subject to the foregoing and in reliance thereon, it is our opinion
that the Shares have been duly authorized by all necessary corporate action on
the part of the Company; upon the issuance and delivery of the Newly Issued
Shares in accordance with such authorization, the payment for the Company Shares
as contemplated in the Registration Statement, the countersigning of the
certificate or certificates representing the Newly Issued Shares by a duly
authorized signatory of the registrar for the Company's Common Stock, and the
transfer of an amount representing the par value of the Split Shares to the
capital account, the Shares will be validly issued, fully paid and non-
assessable.

          We consent to your filing this opinion as an exhibit to the
Registration Statement and the reference to our firm contained under the heading
"Legal Matters."

                              Very truly yours,

                              /s/ O'Melveny & Myers LLP
                              O'MELVENY & MYERS LLP
