Have a great trip Clancy, wish I could join you. I lived there 10 years and am dying to get back. Sadly, the winds on Oahu are not as good as Maui, especially the last 6 years or so. So if it's a windsurfing vacation you're after, I would definitely recommend getting yourself over to Maui.
OTOH, it can blow nicely on Oahu. Look for 5 o'clock Charly -- the winds at Kailua beach often seemed to back off in the early PM, then come back in the late afternoon. There's not much reef to watch out for, so you can sail till dark. If it blows for a few days in a row (I hope you are so lucky), the swell can kick up nicely in Kailua bay allowing for excellent bump n jump. There's also waves at a few locations around the bay: Castles toward the far end of the beach; the main reef about 1/4 mile offshore; a small reef to the south of "flat island" (works best in N-NNW winds). There is a big reef to get around in the middle of the bay if trying to get to the outer reef, and coral heads to the south of flat island, so watch where the locals go or ask the shop guys if you're going off the beaten path to the wave spots. The main Kailua beach launch is at the north end of the park, there's another next to a boat ramp at the far south end of the park (exit the main parking lot, go down the road toward Lanikai, turn left at stop sign, find small parking lot on the left just before the road turns to the right). Note that Kailua beach has a swimming area marked by buoys. Launch to the north (main launch) or south (boat ramp) of it.
Mokuleia is best in mid afternoon on sunny days as it is helped greatly by a thermal effect. But like the other folks said, there's no shop to rent from up there. There is a significant reef up there close to the shore. So be sure to check with local *wind*surfers before going out. (not kitesurfers as they don't have to worry about the reef as much)
My favorite shop is Naish. They'll have a truck onsite at Kailua everyday with good guys to get you set up. I can also recommend Hawaiian Watersports. They're just around the corner from Naish and also bring gear to the beach. They *might* be a bit cheaper. Close to Kailua beach is Kailua Sailboards and Kayaks in a small shopping plaza one block from the north end of the beach park. They'd be great to rent kayaks from to tool around if there's no wind.
In winter, there won't likely be much south shore wave action, but if you're interested in south shore waves, then I'd recommend looking for Kahala beach park near the Mandarin hotel. There's a friendly group of locals there if it's working. There's also another break across from Hawaii Loa ridge called "Toes". You could easily lose a few toes there

(I have a few scars from there). Diamond Head is an option if you're real good at dodging floating objects (surfers) while sunk to your knees on your waveboard.
Enjoy your stay, wear plenty of sunscreen! Don't be too discouraged if it's cloudy during part of your trip. It is winter there, but it's nowhere near as cold as Utah

Let me know if you need any more details on anything specific.
Aloha,
June