I don't think taking the course is essential. Some people can get by without spending 1200 bucks. If your concern is time, then I would say that you should be prepared to study for hours a day whether or not you take the course. What you should really evaluate, is whether or not you think you can gain something from the actual course that you can't gain from just reading a textbook or studying a review book. I felt like the classes were more a steady way to review with a schedule than a course where they teach you all sorts of things. The usefulness of their methods, tips, and tricks is subjective. Knowing the material is still the most important thing.
If you are self motivated and prioritize your studying, I think you can do without the OAT course. It will still take some time studying though. I would recommend, though, that you buy the big Kaplan OAT review notes or the DAT book in stores because it does a good job of reviewing each topic. I also suggest you do whatever practice problems you can get ahold of. The questions really aren't that hard (not nearly as hard as my tests in each class at school) you just have to study for a wide range of problem types and subjects.
Good Luck,
-Jeff
