Oahu vs island

Oahu and island both are nouns.

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

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