theater vs opera house

theater and opera house both are nouns.

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

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

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

theater (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