V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

OSVS1
       Operating System / Virtual System - 1 (OS, IBM, S/370), "OS/VS-1"


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

Operating System/Multiprogramming of Fixed Tasks
OS/MFT
OS/VS1

    (OS/MFT) One of the IBM operating
   systems associated with the IBM 360, released in 1966 and
   targetted at mid-range IBM 360 users (typically 360/40,
   360/50).  OS/MFT was the juinior member of the main 'OS'
   series of IBM operating systems, the other being OS/MVT.
   Smaller 360 mainframes used DOS.

   OS/MFT shared JCL and utilities with OS/MVT but allocated
   memory differently.  In OS/MFT, partitions of memory were of a
   fixed number and size, specified by the generation and
   configuration of the operating system.  As this class of
   mainframe had typically less than 512K of main memory,
   partitions tended to be about 128K.

   With the advent of Virtual Storage and the System 370's
   Dynamic Address Translation (DAT), OS/MFT was improved to
   become OS/VS1, taking account of virtual storage in a single
   partition up to 16MB.

   (1999-01-22)