roar vs hollo

roar and hollo both are nouns.

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

As nouns, roar and hollo are synonyms defined as:

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

As verbs, hollo is a hypernym of roar; that is, hollo is a word with a broader meaning than roar:

Other hypernyms of roar include call, cry, holler, scream, shout, shout out, squall, yell.

roar (noun)	hollo (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)	hollo (verb)
laugh unrestrainedly and heartily	utter a sudden loud cry
make a loud noise, as of wind, water, or vehicles	cry hollo
emit long loud cries	encourage somebody by crying hollo
utter words loudly and forcefully
make a loud noise, as of animal
act or proceed in a riotous, turbulent, or disorderly way