Lived on Oahu for a year 04/05. Tried to sail at diamond head. Every time there was never quite enough wind and TONS of guys surfing, wind or no wind and pretty shallow in some spots. The once in a while that I caught a nice wave I had to gybe off it early because there were just too many surfers down the line. Being in the overwhelming minority as a windsurfer, it was never a good idea to sail there. After a few times I never bothered. I don't know, maybe I just was never there at the right time....
Tried Kailua Bay many times. About half the time my gear was too small (6.6 slalom sail and 105 Hypersonic, me at 175lb). Good place to work on your pumping. Did the long drive to backyards/phantoms on the north shore. Again there was usually not enough wind. Huge waves when you do get out there for sure. Sometimes on the inside there was just no wind, a strong current and incessant white water that it took forever to recover my rig and get out of there. I would say recovering your gear at big Hookipa is easier than big backyards. And if you think Hookipa is shallow at low tide, this place takes the cake... Sailed Mokuleia a few times. It can get pretty big there but again, light winds, if you do get to sail.
Aside from being spooked by the occasional submarine that crusied by silently, some of the best sailing I had on Oahu was at the mouth of Pearl Harbor. Can be some wind and super flat, like sailing on butter. The slalom set up was a lot of fun on those days. About a half mile to a mile out to sea in a SW direction from the mouth of the harbor was actually the best wave sailing I ever had on Oahu. It's not on the 'map' as far as known wavesailing spots but it was actually better than diamond head. Same orientation, so left foot back. There are breaking waves out there and you can still see the bottom that far out. I had some powered up days there on a 4.7 with waves slightly overhead. If you are sailing there alone and break down be prepared for a long swim back because there is nobody to help you because absolutely nobody sails there.
Maui is windier and the driving distances are much shorter. Kihei to Kanaha/Hookipa is 20-25min while the sailing spots on Oahu are an hour from each other at best. And you may not get to sail. There are a few other spots on Oahu. East some way past diamond head there is a grassy park I forget the name of. Once in a while you'll see another guy out. As far as I remember the launch out of any part of the SE shore of Oahu is sharp and shallow. Ended up making weekend trips back to Maui to sail.
Other than this, Oahu is a great place to windsurf.