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

ZOS
       Z/Operating System (IBM, Java, OS), "z/OS"


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

z/OS

    An operating system from IBM.
   According to IBM, it is a secure, scalable, high-performance
   enterprise operating system on which to build and deploy
   Internet and Java-enabled applications, providing a
   comprehensive and diverse application execution environment.

   (http://ibm.com/servers/eserver/zseries/zos/).

   (2008-01-22)