onion roll vs bun

onion roll and bun both are nouns.

Word	Noun	Adjective	Verb	Adverb
onion roll	Yes	No	No	No
bun	Yes	No	No	No

As nouns, bun is a hypernym of onion roll; that is, bun is a word with a broader meaning than onion roll:

Other hypernyms of onion roll include roll.

onion roll (noun)	bun (noun)
yeast-raised roll flavored with onion	small rounded bread either plain or sweet