grad vs old boy

grad and old boy both are nouns.

Word	Noun	Adjective	Verb	Adverb
grad	Yes	No	No	No
old boy	Yes	No	No	No

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

grad: a person who has received a degree from a school (high school or college or university)
old boy: a former male pupil of a school

Other hyponyms of grad include alumnus, alumna, Ivy Leaguer.

grad (noun)	old boy (noun)
a person who has received a degree from a school (high school or college or university)	a vivacious elderly man
one-hundredth of a right angle	a former male pupil of a school
	a familiar term of address for a man