Fortunately, the universities are compiling waiting lists, so if your course selection is full, you may want to go ahead and sign up. Just don't send your $35 registration fee. If an opening becomes available for you, the charge will be assessed at that time. I am sending my registration in for the Pre-AP English for Middle School today, and hoping I receive a spot.
So far, the full courses at OU include AP English Language and Literature, and Pre-AP English for High School and for Middle School.