old master vs maestro

old master and maestro both are nouns.

Word	Noun	Adjective	Verb	Adverb
old master	Yes	No	No	No
maestro	Yes	No	No	No

As nouns, maestro is a hypernym of old master; that is, maestro is a word with a broader meaning than old master:

Other hypernyms of old master include master, painter.

old master (noun)	maestro (noun)
a great European painter prior to 19th century	an artist of consummate skill