Because the OKW GV require fewer points than any other GV, you are competing with lots of members. And OKW members may book them up before you get a chance.
Forgot to add, make a reservation at your home resort at 11 months out. Since your home resort probably takes more points, just get a two bedroom and a studio (it might be about the same number of points or close to it). Then at 7 months out, see if you can get the GV at OKW. Just cancel your reservations and book the GV at that time. That way you will at least have somewhere to stay if you can't get the GV. You may have to worry about banking points, though, if your first reservation uses more than the GV.