I'm glad to see so many people made it over here to give answers. We are members at OKW and have stayed many times. Be sure to say you need first floor for Medical Reasons like everyone else said and you should have no problems getting it. When you get your confirmation letter, make sure it says First floor for Medical Reasons on the letter. We have been told that they fill the medical requests first and then fill the general requests for first floor after that.
Don't assume that telling them you have an ecv will make them give you a first floor room. Once when we made reservations, we told them we had a DD with a wheelchair and somehow it didn't register to the person putting it in the computer that we would require first floor; she put it as a preference. We ended up arriving at midnight to a second floor room. Since then, we have made sure our confirmation letter says the right thing and we haven't had a problem.