musical arrangement vs opus

musical arrangement and opus both are nouns.

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

As nouns, opus is a hypernym of musical arrangement; that is, opus is a word with a broader meaning than musical arrangement:

musical arrangement: a piece of music that has been adapted for performance by a particular set of voices or instruments
opus: a musical work that has been created

Other hypernyms of musical arrangement include composition, musical composition, piece, piece of music.

musical arrangement (noun)	opus (noun)
a piece of music that has been adapted for performance by a particular set of voices or instruments	a musical work that has been created