I can't tell you whether Kaplan would have boosted my scores or not, but I can tell you about my experiences with Kaplan and my overall impression based on my discussions with people who have actually taken the class.
Before I even cracked open a review book or took a quick peek at old class notes, I attended a free Kaplan practice test on my college campus. I was only one of two people taking a practice test, so I was able to pick the brain of the person administering the test. She was a pre-dental student who had aced the DAT after going through a Kaplan course. She told us all about how she didn't know what she would have done without it, and how there was no better way to make sure you are prepared for everything. She also said that Kaplan teaches you a few "secret strategies" to getting the right answer without knowing it. Then, test time. No surprise, I spent the next few hours in a panic and turned in my test feeling horrible about my performance. She went through a few problems with us to show us how Kaplan would teach us to think, which was kind of an anti-climax for me; I was already thinking that way. Once we turned in the test, she told us we could get $300 off the class if we signed up that day. They were very inflexible about giving an extension so that we could sleep on it. I didn't have the money so I couldn't get the course.
I spent the next few days stressing until I got my scores from them: AA of 290. I was completely bummed out and was regretting not signing up for the class. Those "secret strategies" and expert tips and tricks would have been really handy. But, when I got out for winter break a few weeks later, I just hunkered down with some AP review books, class notes, and a skinny Kaplan OAT book and in 3 weeks I was able to bring my real score up to an AA of 350.
I've talked to quite a few people who swear by the Kaplan course, but I really don't think you need it. Their materials are GREAT - I absolutely loved the Kaplan practice test book (about $20). $20 is much better than their class fees! The way I see it, if I was able to improve so much on my own in just 3 weeks, I think I could have done even better if I had invested the amount of time it takes to get through a Kaplan class. Granted, the Kaplan course doesn't include 6 hours of studying a day like my 3-week cram fest did, but you get my point. I really felt like the people admisnistering the practice test at my college were trying to capitzalize on my post-test freak-out session. I can't blame them - that's business! But they make you feel like you need it and you really don't.
Anyway, I don't want to sound like I'm completely against the course. In fact, if I had the time and money I would have taken it myself. But I just want to emphasize that it is possible to do it on your own and that there really aren't any secret strategies that you haven't learned in your years at college and while taking the SAT. What's more, it REALLY depends on what kind of score you need on the OAT. If you have a stellar GPA and application, you might just want to get through the OAT without bombing too bad. If you are counting on the OAT to toughen up your application, maybe Kaplan would be more worth it.
OK, sorry for rambling! I just remember that feeling of panic about not taking Kaplan and I don't want anyone else going through it!