Source : Free On-Line Dictionary of Computing

Tape Operating System
     
         (TOS) An {IBM} {operating system} for
        {System 360}, used in the early days around 1965 to support
        the {IBM 360} model 30 et al.  TOS was a predecessor to IBM's
        {Disk Operating System}.
     
        TOS died out really early as disks such as the 2311 and 2314
        became common with the {IBM 360} whereas thet had been a real
        luxury on the {IBM 7090}.
     
        [Relationship to {BOS}?]
     
        (1999-01-20)