Oahu Island vs Oahu

Oahu Island and Oahu both are nouns.

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

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

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

Oahu Island (noun)	Oahu (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