I struggled with this, too, on my 20 year old Oreck when the motor gave out. Once you remove the nut, the fan really is only just "stuck" on there. I put WD-40 on the shaft, put I still could not pull it apart with hand strength. Then I stuck two small screwdrivers, one on either side, between the fan and the flat metal plate and pried and it popped off after applying pretty good, but not excessive, force. Bottom line: you need something in that small gap between fan blade and metal disk to pry it off.
fivester, January 2010