The Collaborative International Dictionary of English v.0.48:

Opus \O"pus\, n.; pl. Opera. [L. See Opera.]
   A work; specif. (Mus.), a musical composition.
   [1913 Webster]

   Note: Each composition, or set of pieces, as the composer may
         choose, is called an opus, and they are numbered in the
         order of their issue. (Often abbrev. to op.)
         [1913 Webster]

   Opus incertum. [L.] (Arch.) See under Incertum.
      [1913 Webster] Opuscle

WordNet (r) 3.0 (2006):

opus
    n 1: a musical work that has been created; "the composition is
         written in four movements" [syn: musical composition,
         opus, composition, piece, piece of music]

The Free On-line Dictionary of Computing (19 January 2023):

Opus

    A Honeywell operating system promised
   as a sop to customers after canning Multics in 1985.  Opus
   was to provide everything Multics had and more, plus total
   compatibility with the Level 6/DPS6 operating system.

   "Opus" was a code name, the system was officially named VS3
   (short for HVS R3 or Honeywell Virtual System Release Three).
   It was to run on the DPS6-plus hardware known internally as
   the MRX and HRX, and be all things to all people.

   The hardware was a dud (though it did run the native DPS6
   software just fine), and the goal was, shall we say,
   ambitious.  The effort was cancelled by Bull in 1987, in
   favor of another project going on in France.