How many people are you looking at because if its more than 12 I think you have to call group reservations. Also I think you have to wait 180 days out like regular reservations.
Your planner can do it for you...as of now you have to wait to book anything after March 22nd on Dec 22nd. You may be able to book it now if it is a group dining but I could be wrong.
I had my rehearsal dinner at Ohana my planner set it up for me. There were about 40 people and if you ask you might be able to sit near the castle windows.