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

OPS5

    A programming language for rule-based production
   systems.  A rule consists of pre-condition(s) and a resulting
   action.

   The system checks its working memory to see if there are
   rules whose pre-conditions are satisfied, if so, the action in
   one selected satisfied rule is executed.

   There is a public domain implementation of an OPS5
   interpreter written by Charles L. Forgy 
   in 1977.  It was first implemented in Lisp and later in
   BLISS.  It was also ported to Common Lisp by George Wood
   and Jim Kowalski.

   CLIPS is a language for writing expert systems, with some
   of the capabilities of OPS5.

   See also C5, OPS83, OPS4, OPS5+, OPS83.

   Inference Engine Tech, Cambridge MA.

   An OPS5 interpreter in Common LISP
   (ftp://ftp.wustl.edu/mirrors/Unix-c/languages/ops5).

   A version by Mark Kantrowitz
   (ftp://ftp.cs.cmu.edu/afs/cs.cmu.edu/user/mkant/Public/Lisp/).
   .

   ["Programming Expert Systems in OPS5", L. Brownston et al, A-W
   1985].

   ["An OPS5 Primer", Sherman et al, comes with OPS5 for DOS].

   ["Rule-Based Programming in the Unix System", G.T. Vesonder,
   AT&T Tech J 67(1), 1988].

   (1995-08-18)