roar vs bellow

roar and bellow both are nouns.

Word	Noun	Adjective	Verb	Adverb
roar	Yes	No	Yes	No
bellow	Yes	No	Yes	No

As nouns, roar and bellow are synonyms defined as:

Other synonyms of roar include bellowing, holla, holler, hollering, hollo, holloa, roaring, yowl.

As verbs, roar and bellow are synonyms defined as:

roar (noun)	bellow (noun)
a very loud utterance (like the sound of an animal)	a very loud utterance (like the sound of an animal)
a deep prolonged loud noise
the sound made by a lion

roar (verb)	bellow (verb)
laugh unrestrainedly and heartily	shout loudly and without restraint
make a loud noise, as of wind, water, or vehicles	make a loud noise, as of animal
emit long loud cries
utter words loudly and forcefully
make a loud noise, as of animal
act or proceed in a riotous, turbulent, or disorderly way