sextet vs opus

sextet and opus both are nouns.

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

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

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

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