We used Makani Kai when we vacationed on Oahu 1-1/2 years ago. I chose them because they go beyond the safety requirements to earn the highest possible safety accreditation. This is the company that Hawaii Five-0 uses when they need a helicopter. Another plus is that once a week, they have tours that begin and end at the Ko'Olina Marina, so you don't have to go to the airport.
Their flights will hold up to 6, but no one else booked the same time we did, so we had a private flight! Unfortunately for us, the eastern side of the island was very rainy. We made it to just past Hanauma Bay when turbulence became too rough too continue around the island. Visibility was impossible by then too. Our pilot turned around and flew up the middle of the island, over the Dole Plantation, up to the coast and then turned left to go around the northwest tip of the island. That's an area that is not usually covered on the tour and it's gorgeous! It is also amazing to see Aulani from the sky. It was a once in a lifetime experience! I'm glad we did it!