halloo vs holler

halloo and holler both are nouns.

Word	Noun	Adjective	Verb	Adverb
halloo	Yes	No	Yes	No
holler	Yes	No	Yes	No

As verbs, holler is a hypernym of halloo; that is, holler is a word with a broader meaning than halloo:

Other hypernyms of halloo include call, cry, hollo, scream, shout, shout out, squall, yell.

halloo (noun)	holler (noun)
a shout to attract attention	a very loud utterance (like the sound of an animal)
	a small valley between mountains

halloo (verb)	holler (verb)
shout `halloo', as when greeting someone or attracting attention	complain
urge on with shouts	utter a sudden loud cry
	shout out