operating room vs operating theater

operating room and operating theater both are nouns.

Word	Noun	Adjective	Verb	Adverb
operating room	Yes	No	No	No
operating theater	Yes	No	No	No

As nouns, operating room and operating theater are synonyms defined as:

operating room and operating theater: a room in a hospital equipped for the performance of surgical operations

Other synonyms of operating room include operating theatre, OR, surgery.

operating room (noun)	operating theater (noun)
a room in a hospital equipped for the performance of surgical operations	a room in a hospital equipped for the performance of surgical operations