octet vs opus

octet and opus both are nouns.

Word	Noun	Adjective	Verb	Adverb
octet	Yes	No	No	No
opus	Yes	No	No	No

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

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

octet (noun)	opus (noun)
a musical composition written for eight performers	a musical work that has been created
eight people considered as a unit
a set of eight similar things considered as a unit
eight performers or singers who perform together
the cardinal number that is the sum of seven and one