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.
OAT DESTROYER helps me a lot also OAT sample from opted website. The opted exam is a very good estimate to the actuale OAT, beleive it or not I got the same scores on both the opted exam and on the actual OAT. Good luck.
I think that Kaplan review book and Achiever will help you to feel confident on the actual exam as happend with me. Yes , they are different from actual exam but they help to study. Achiever is difficult for somebody who does not familiar with material. As long as you start to learn more it will be very useful tool for reviewing any subject. Some questions on the actual exam can be difficult to answer without solid knowledge of subject (any). Do not relay on people's opinion about OAT. Just be prepared to read questions and answer quickly on them while you are reading.