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

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "opus":
   Nachtmusik, absolute music, adaptation, air varie, aleatory,
   aleatory music, arrangement, article, artifact, autograph,
   best seller, book, bound book, brainchild, chamber music,
   chamber orchestra, child, classic, coinage, coloring book,
   composition, computer printout, concoction, copy, creation,
   creature, crowning achievement, definitive work, descant,
   distillation, document, draft, edited version, effect,
   electronic music, end product, engrossment, essay, essence, etude,
   exercise, extract, fair copy, fiction, final draft,
   finished version, first draft, flimsy, folio, fruit, great work,
   handiwork, hardback, harmonization, holograph, incidental music,
   instrumental music, invention, issue, juvenile, juvenile book,
   letter, limp-cover book, literae scriptae, literary artefact,
   literary production, literature, lucubration, magnum opus,
   manufacture, manuscript, masterpiece, masterwork, matter, mintage,
   new mintage, nocturne, nonbook, nonfiction, notebook, novel,
   oeuvre, offspring, opera, opuscule, opusculum, orchestration,
   original, origination, outcome, outgrowth, paper, paperback,
   parchment, penscript, picture book, piece, piece of writing, play,
   playbook, pocket book, poem, prayer book, printed matter, printout,
   product, production, program music, psalmbook, psalter,
   publication, reading matter, recension, result, ricercar, score,
   screed, scrip, script, scrive, scroll, second draft, serial,
   sketchbook, soft-cover, sonata, sonatina, songbook, standard work,
   storybook, string orchestra, string quartet, study,
   the written word, theme and variations, title, tome, trade book,
   transcript, transcription, trio, typescript, variation, version,
   volume, work, writing