tso

from The Free On-line Dictionary of Computing (8 July 2008)

Time Sharing Option
TSO

   <operating system> (TSO) System software from {IBM} that
   provides {time-sharing} on an IBM {mainframe} running in an
   {MVS} environment.

   (2003-08-15)

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

TSO
       Telecommunications Service Order

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

TSO
       Time Sharing Option