shout out vs holler

shout out is not a noun while holler is a noun.

Word	Noun	Adjective	Verb	Adverb
shout out	No	No	Yes	No
holler	Yes	No	Yes	No

As verbs, shout out and holler are synonyms defined as:

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

shout out (noun)	holler (noun)
	a very loud utterance (like the sound of an animal)
	a small valley between mountains