As is the case with all of the Hawaiian Islands, there are many areas of Oahu that can only be viewed from the air. Even places that you're used to seeing from the ground gain a whole new perspective when viewed from above.
From a helicopter you'll be able to see the oil slick that still gently flows from the hull of the sunken USS Arizona and you'll really be able to appreciate the beauty of the sandbars off of Kaneohe on Oahu's eastern shore.
I have flown with two companies on Oahu. Paradise Helicopters flies out of the Turtle Bay Resort on Oahu's North Shore. Makani Kai Helicopters flies out of the Honolulu International Airport and the Ko Olina Resort and offers free roundtrip transportation from Waikiki.