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

OS/390

    An IBM mainframe operating system,
   featuring integrated MVS, UNIX, LAN, distributed
   computing and application enablement services through its
   base elements.  These base services enable open, distributed
   processing and offer a foundation for object-ready application
   development.  The OS/390 base includes a Communication
   Server that includes VTAM, the VTAM AnyNet feature,
   TCP/IP and TIOC.  It provides SNA (3270), APPC,
   High Performance Routing, ATM support, sockets and
   RPC.

   OS/390 is basically rebranded, repackaged MVS/OE, CMOS
   processors, RAMAC disk arrays and open systems extension
   to networking in VTAM, the principle being that if you can't
   compete, rebrand what you have and tell everyone it's
   something new.

   (http://204.146.133.206/os390/index.html).

   (1999-01-20)