I was on Oasis last week, and was surprised how dressed up people were on formal night. Sure, there were some (a few?) folks who wore shorts and tee shirts, but the majority vibe was that people were "observing the occasion" - with everything from your plan of nice slacks, dress shirt & tie all the way up to very showy gowns and tuxedos. I wore a fairly non-formal embroidered cotton sheath dress, gold sandals and some blingy jewelry and didn't feel at all overdressed. OTOH, I suspect that the fact that many people drive to sail on Oasis means they can bring more luggage - I know I certainly did.