The Free On-line Dictionary of Computing (19 January 2023):

Operating System/360
OS/360

    (OS/360) An operating system developed by
   IBM for their System/360 computer (announced in 1964).

   After this experience, Frederick P. Brooks wrote his famous
   book, The Mythical Man-Month, giving OS/360 as an example of
   the second-system effect.

   [Features?  Relatonship to DOS/360?]

   (2001-03-23)