OK vs all right

OK is a noun but all right is not a noun.

Word	Noun	Adjective	Verb	Adverb
OK	Yes	No	No	Yes
all right	No	Yes	No	Yes

As adverbs, OK and all right are synonyms defined as:

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

Other synonyms of OK include alright, fine, very well.

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

OK (adjective)	all right (adjective)
	being satisfactory or in satisfactory condition

OK (adverb)	all right (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