introit vs opus

introit and opus both are nouns.

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

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

introit: a composition of vocal music that is appropriate for opening church services
opus: a musical work that has been created

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

introit (noun)	opus (noun)
a composition of vocal music that is appropriate for opening church services	a musical work that has been created