maestro vs old master

maestro and old master both are nouns.

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

As nouns, old master is a hyponym of maestro; that is, old master is a word with a more specific, narrower meaning than maestro:

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