I dive mainly with Island Divers Hawaii, they have shops at Hickham Airforce base and in Hawaii Kai. Their boat is located at the Hawaii Kai shopping center on Keahole St. Their boat is called the Sea Fox and will handle about 14-18 divers. They do pickups from hotels and drive you back after you return from diving. This will be south shore diving so hopefully the swells will be down enough for you to have a great boat ride out and back. Once you’re under, it doesn’t matter (boat dives). Stay away from the north shore - unless you want to surf there.
All dive shops mentioned by the others are good also. Check out their websites as they all visit the same dive sites.
If you want to do a beach dive, try Electric Beach on the west side of Oahu. But only if the swells aren’t big! I usually pay the homeless guy to watch my car $2-5 or give him some dry food. If the surf is big every where, sometimes this is the only beach diveable place. Boat diving is fine but for December the boats will go to the leeward sides of the island.
Go to www.shorediving.com for more info on the beach dives for Oahu and read what other divers commented on the site but pay attention to when the comment was made.
Next time plan your dive vacation during late May thru end of September for the North Shore dives, this area is the best the island has to offer.
Sharks Cove, 3 Tables, The Trench are just a few to try.
Good luck,
Charlie