island vs Oahu Island

island and Oahu Island both are nouns.

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

island (noun)	Oahu Island (noun)
a zone or area resembling an island	an island of central Hawaii (between Molokai and Kauai); the chief island of the state
a land mass (smaller than a continent) that is surrounded by water