musical composition vs opus

musical composition and opus both are nouns.

Word	Noun	Adjective	Verb	Adverb
musical composition	Yes	No	No	No
opus	Yes	No	No	No

As nouns, musical composition and opus are synonyms defined as:

Other synonyms of musical composition include composition, piece, piece of music.

musical composition (noun)	opus (noun)
a musical work that has been created	a musical work that has been created