H.I. vs Oahu Island

H.I. and Oahu Island both are nouns.

Word	Noun	Adjective	Verb	Adverb
H.I.	Yes	No	No	No
Oahu Island	Yes	No	No	No

H.I. (noun)	Oahu Island (noun)
a group of volcanic and coral islands in the central Pacific	an island of central Hawaii (between Molokai and Kauai); the chief island of the state