EXHIBIT 5.1 [LETTERHEAD OF GRAY, PLANT, MOOTY, MOOTY & BENNETT, P.A.] LINDLEY S. BRANSON 612 343-2827 January 29, 1999 Online System Services, Inc. 1800 Glenarm Place Suite 800 Denver, CO 80202 RE: Form S-3 Registration Statement Ladies/Gentlemen: This opinion is furnished in connection with the registration, pursuant to the Securities Act of 1933, as amended, of up to 840,000 shares of common stock, no par value (the "Shares"), of Online System Services, Inc. (the "Company" or "OSS") issuable upon (i) conversion of the Company's Series C Preferred Stock (the "Series A Preferred Stock") and (ii) exercise of outstanding transferable warrants of OSS which may sold from time to time by various selling shareholders for their own account. We have acted as counsel to the Company in connection with the preparation of the Form S-3 Registration Statement (the "Registration Statement"). We have examined the Articles of Incorporation, as amended, the Bylaws of the Company, such records of proceedings of the Company as we deemed material and such other certificates, records and documents as we considered necessary for the purposes of this opinion. Based on the foregoing, we are of the opinion that the Shares will be, when issued, legally issued, fully paid and non-assessable securities of the Company. We understand that this opinion is to be issued in connection with the Registration Statement. We consent to a filing of a copy of this opinion with the Registration Statement. Very truly yours, GRAY, PLANT, MOOTY, MOOTY & BENNETT, P.A. By /s/ Lindley S. Branson ----------------------------------- Lindley S. Branson