septet vs opus

septet and opus both are nouns.

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

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

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

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