opus vs piece of music

opus and piece of music both are nouns.

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

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

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

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