Oahu vs Oahu Island

Oahu and Oahu Island both are nouns.

Word	Noun	Adjective	Verb	Adverb
Oahu	Yes	No	No	No
Oahu Island	Yes	No	No	No

As nouns, Oahu and Oahu Island are synonyms defined as:

Oahu and Oahu Island: an island of central Hawaii (between Molokai and Kauai); the chief island of the state

Oahu (noun)	Oahu Island (noun)
an island of central Hawaii (between Molokai and Kauai); the chief island of the state	an island of central Hawaii (between Molokai and Kauai); the chief island of the state