Source : Free On-Line Dictionary of Computing

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/)}.