I was overwhelmed by the OAT prep books and went to the library and got the (ochem, bio, physics etc) for dummies books or any high school subject books. It was dumb down enough that I could get through it quickly and get the basics down and easy reference. I then went back to Kaplan books to cover more material and worked on practice problems. To do well you need to understand concepts or pure memorization. I didn't get 350+ but I improved significantly with this method.
If you have the money get the OAT Achiever. The problems may be harder and probably irrelevant on the OAT but will point out your weakness and prepare you for the computerize test. My problem was I never took a computerized test and spent too long on one problems and could have used it to do problems I actually knew. Use the Achiever to get use to managing your time.