roar vs hollering

roar and hollering both are nouns.

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

As nouns, roar and hollering are synonyms defined as:

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

roar (noun)	hollering (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)	hollering (verb)
laugh unrestrainedly and heartily
make a loud noise, as of wind, water, or vehicles
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