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

Basic Operating System

    (BOS) An early [when?] IBM operating
   system.

   According to folklore, BOS was the predecessor to TOS on the
   IBM 360 and it was IPL'd from a card reader.  It may
   have been intended for very small 360's with no disks and
   limited tape drives.

   BOS died out really early [when?] as disks such as the 2311
   and 2314 became common with the IBM 360, whereas disks had
   been a real luxury on the IBM 7090.

   (1999-01-29)