You will be at absolutely no disadvantage applying in September. That's still fairly early in the application cycle. By the way, you can submit your application before your OAT scores are in, and then it can be updated once you obtain the scores (be aware of the 30 day waiting period for scores to become official).
The important thing for you, then, is make sure you only have to take the OAT once; i.e. do as well as you can on it the first time. I personally took my OAT exam last August, and hence was able to spend the entire summer preparing for it, and that was one of the smartest decisions I've ever made.