I used OAT Achiever which gave tests in the same exact format as the OAT. It also timed me so I was working under pressure.
I also used Kaplan books as well as the older Kaplan practice test books I found at my local library.
There is also Chad's quizzes available for purchase or you can start using (his bio or chem ones I think are free!). I personally didn't use them much but other people have commented they were helpful!