cooperate (to work or act together toward a common end or purpose)
magnum opus (a great work, especially a literary or artistic masterpiece)
oeuvre (a work of art; the sum of the lifework of an artist, a writer, or a composer)
opera (a plural of opus)
operate (to perform a function; work)
operose (involving great labor; laborious)
opus (a creative work, especially a musical composition numbered to designate the order of a composer's works)
opuscule (a small, minor work)
Definitions are taken from:
The American Heritage® Dictionary of the English Language, Third Edition copyright © 1992 by Houghton Mifflin Company.
Electronic version licensed from InfoSoft International, Inc. All rights reserved.
Webster's Third New International Dictionary, Unabridged. Merriam-Webster, 2002.
http://unabridged.merriam-webster.com (30 Sep. 2006).
English-Word Information: Word Info about English Vocabulary (wordinfo.info).