OK vs alright

OK is a noun but alright is not a noun.

Word	Noun	Adjective	Verb	Adverb
OK	Yes	No	No	Yes
alright	No	Yes	No	Yes

As adverbs, OK and alright are synonyms defined as:

OK and alright: an expression of agreement normally occurring at the beginning of a sentence

Other synonyms of OK include all right, fine, very well.

OK (noun)	alright (noun)
an endorsement
a state in south central United States

OK (adjective)	alright (adjective)
	nonstandard usage

OK (adverb)	alright (adverb)
an expression of agreement normally occurring at the beginning of a sentence	an expression of agreement normally occurring at the beginning of a sentence
	without doubt (used to reinforce an assertion)
	in a satisfactory or adequate manner