The Free On-line Dictionary of Computing (30 December 2018):

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)