If you're taking the OAT in August, then there really isn't a reason to sign up for it yet. You are correct that it would be wise to sign up for it early, but if you signed up in June that would still be considered early by most standards.
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.