theatre vs opera house

theatre and opera house both are nouns.

Word	Noun	Adjective	Verb	Adverb
theatre	Yes	No	No	No
opera house	Yes	No	No	No

As nouns, opera house is a hyponym of theatre; that is, opera house is a word with a more specific, narrower meaning than theatre:

theatre: a building where theatrical performances or motion-picture shows can be presented
opera house: a building where musical dramas are performed

theatre (noun)	opera house (noun)
a building where theatrical performances or motion-picture shows can be presented	a building where musical dramas are performed
the art of writing and producing plays
a region in which active military operations are in progress