Bio you could probably teach yourself, orgo in my opinion is a little more complicated to learn and study on your own but if you think you can do it, then more power to you. If you have the money and time and study and take it in August, it wouldn't hurt. However if you haven't taken the subjects before, I would suggest taking a few practice tests before signing up for the real test and seeing how you do. It's not terribly hard. December is still relatively early to submit your scores. Schools say you're in good shape if you have everything submitted by Dec-Jan (my roommate interviewed in January and was accepted). You should still submit your application in July though. Your OAT won't be the only part of your application, and by submitting early you will have a head start. Schools will already be looking at the rest of your application. It's preferable to have schools waiting on 1 part of your application than waiting on the whole thing. Also there will be way more people submitting applications in December than July so the pool of applicants that you'll be competing against will be much larger.
