I signed up for my 2nd OAT like...3 weeks before I had intended to take it. It was around Thanksgiving and I think I wanted to try to do it like...the day before Thanksgiving but everything was booked 1 week before and 1 week after the holiday. I ended up scheduling for about a week before Thanksgiving cutting my remaining study time down to 2 weeks. Yuck. Anyways, so like..3 days before my scheduled date, I freaked out and was like "I'M NOT READY" and pushed it back another week without any problems. Though there previously wasn't a spot available that week, so I'm sure someone had canceled which allowed a space to open up for me.
So take-home-note: Register at least a month before so you can have better control over when you take it. But don't be afraid to push it back if you're not ready. I think there's only a $20 resched. fee.