Red River of the South vs OK

Red River of the South and OK both are nouns.

Word	Noun	Adjective	Verb	Adverb
Red River of the South	Yes	No	No	No
OK	Yes	No	No	Yes

Red River of the South (noun)	OK (noun)
a tributary of the Mississippi River that flows eastward from Texas along the southern boundary of Oklahoma and through Louisiana	an endorsement
	a state in south central United States

Red River of the South (adverb)	OK (adverb)
	an expression of agreement normally occurring at the beginning of a sentence