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

CSO

   Campus Phone Book software developed for, and originally used
   at, the Computer Services Office of the University of Illinois
   at Urbana-Champaign.  The server software is known as "qi" and
   the client is "ph".  Recent versions of the software refer to
   CCSO (Computing & Communications Service Office).

   (ftp://uxc.cso.uiuc.edu/).