Take practice tests. Whenever you get something wrong, read a chapter in a textbook about the question and understand what you got wrong. Do this for every question in the book. When you are done, take the practice test again and see which ones you continue to get wrong, and read a chapter again for any you continue to get wrong.
For QR, practice doing math quickly. This is highschool stuff you can definately get a 400, but most do not because you have such short time constraints. Practice without a calculator and work fast. After you finish a round of tests, do it again and see how fast you can do it while getting it correct (don't memorize what the answers were, and even if you do, still do the calculations to show your work). If you can't do shorthand math, learn how to cancel large factors, estimate, work with scientific notation, etc, because there is always an easy way to solve one of the OAT math questions.
The only study material I used were my textbooks and a book of 3 complete OAT tests. I don't know about the quality of the KAPLAN materials, but I think textbooks usually do a great job at explaining things at the detail you need to know. I probably studied only 80 hours total for the exam, and I was able to go through my textbooks twice (at least the parts I needed to review because I had gotten those questions wrong).
Biology I didn't study for because it is very broad and so I just relied on my education from pre-reqs (as long as you got A's/B's in them, you should at least get a 340 on bio without extra studying). If there are subjects you know you suck at and are tested on the OAT, be sure to study those (you'd probably miss them anyways while doing my "get a question wrong, read a chapter method"), for example, I really forgot everything I knew about pregnancy and sex so I spent time on those.
Honestly though, the OAT isn't really that difficult. It's okay to be nervous because afterall it's probably the most important factor on your application, but it's just a brisk test that you will definately do well on
if you feel comfortable on the material. And why shouldn't you? You learned from your mistakes and you reread chapters and you learned to do math quickly. Lastly, don't let low test scores on practice tests scare you too much. Learn from your mistakes (whether it was not knowing or you got tricked).
And you definately have time to reread textbooks. After having taken the pre-requisites, you should be able to quickly review the textbooks in a very fast manner.
If you need the drive, pretend you are Elle from Legally Blonde and you need to get at least a 375 on the LSAT or you won't get into Harvard Law and you're going to lose your bf/gf, who in the end was really a jerk and you'll fall in love with a TA. That's what I did and I ended up getting a 400/380.
There's no magic or secret tip to easily get a top notch score. You've got to take the effor to study, and studying means reading and practicing. Just do what you've got to do. If you're freaking out, worrying won't help you, and you can only ease your tension by being productive and studying (and actually understanding!!!). If you honestly evaluate your progress and you feel totally unprepared, then I advise that you delay your test date until you feel more confident, not only in yourself but in the subject matter.
. You have to practice without a calc, ESPECIALLY if you have become dependent! Use the practice test to gauge where you stand...and round like there is no tomorrow.