I dove Oahu a 10 years ago and wasn't impressed with the diving. I only did four dives on that trip.
This trip to Maui, the diving is excellent. I did Molokini four times and I'm not tired of it, also did four beach dives. Molokini is fish bowl diving and we saw a huge Manta on three dives, and a medium sized Manta on one of the beach dives. The beach dives had just OK vis but the Molokini dives had exceptional vis. There is pretty coral not the huge sponges and formations like the Caribbean, but lots and lots of fish. Twice we saw white tips, once in a cavern, once in an overhang. Lots of turtles, and they don't mind you, they actually get kinda close to you.
I haven't dove the big island so I can't help you there, but I'm loving Maui. It has good food, great diving, excellent golfing, lush landscaping (near Wailea and Hana) and lots of places to explore. :thumbsup: