I'm surprised I haven't specifically answered this already! A lot of people I knew in undergrad who were applying to optometry school took the Kaplan prep course for the OAT. To me it seemed pretty costly, but they seemed to be really glad they took it. I actually purchased the Kaplan course materials from a friend who had taken the course the year before, and found those to be all I really needed. The books are like mini-textbooks that teach you the basics of everything you need to know for the OAT. If you are motivated enough to make yourself study, that's a good, cheaper alternative. I also studied from some of the class notes and textbooks for classes I had already taken, and I recall that helping a bit too. I recommend taking some practice tests so you can get a feel for how the test works, and so you aren't going in having no idea what to expect.
All that being said, I didn't actually do that spectacularly on the OAT. I kind of wound up cramming for it since I was in between whether to give up and take a year off before applying or to go for it anyway and see what happens. So I think that makes me a model example of how getting into optometry school isn't all about the OAT. However, if you have the time and the willpower, I suggest studying strong and taking the OAT early. That way, if you wind up not happy with your score, you aren't stuck with it and you have time to retake it. Just don't forget to take the time to make yourself a well-rounded applicant.