You'll probably see this a lot if you search this thread, but everyone seems to agree that the sample OAT on the opted website is the closest to the actual OAT -- as it should be, because it's actually from the same company! The other OAT study materials are from other companies which are trying to copy OAT-ish questions.
I personally experimented with the thin Kaplan book (the one that you can get without taking the review course), but those questions were SIGNIFICANTLY more difficult than the actual OAT.
I recommend just reviewing the general equations and big ideas from your science courses (I really liked the summaries found in AP subject review books), and take the opted exam to work on pacing.
Good luck!