Oahu Island vs H.I.

Oahu Island and H.I. both are nouns.

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

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