island vs Oahu

island and Oahu both are nouns.

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

island (noun)	Oahu (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