halloo vs hollo

halloo and hollo both are nouns.

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

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

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

halloo (noun)	hollo (noun)
a shout to attract attention	a very loud utterance (like the sound of an animal)

halloo (verb)	hollo (verb)
shout `halloo', as when greeting someone or attracting attention	utter a sudden loud cry
urge on with shouts	cry hollo
	encourage somebody by crying hollo