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

Oracle Parallel Server
Parallel Server Option
PSO

    (OPS) An Oracle configuration that allows for
   multiple Oracle servers running on seperate computers to
   access the same database files simultaneously.

   Normally used for high availability, running parallel
   servers can improve performance by spreading out CPU load,
   however, it requires data to be partitioned correctly.

   This feature is enabled with the Parallel Server Option (or
   "PSO").

   Oracle Parallel Server Option FAQ
   (http://orafaq.com/faqops.htm).

   (2003-04-05)