piece of music vs opus

piece of music and opus both are nouns.

Word	Noun	Adjective	Verb	Adverb
piece of music	Yes	No	No	No
opus	Yes	No	No	No

As nouns, piece of music and opus are synonyms defined as:

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

piece of music (noun)	opus (noun)
a musical work that has been created	a musical work that has been created