hi! i think the makers of the OAT usually have sample tests that they can mail you. here's the link for their website:
http://www.opted.org/info_oat.cfm
and a bit of info they posted: "You can request a free OAT candidate guide information booklet and a test application form by contacting the Optometric Admission Testing Program, 211 East Chicago Avenue, 6th Floor, Chicago, IL 60611-2678, (800)232-2159. Only the printed edition of the OAT Candidate Guide contains samples of the four tests used in the Optometry Admission Testing Program."
There's also a CD program known as TopScore Pro that you can purchase online that has full-length practice exams - i thought they were pretty helpful, but a little harder than the real thing (which is good - it'll help you find your weak points). it's also great for making sure you time yourself - the reading comprehension and quantitative reasoning sections are definitely where you might feel time pressured.
if you sign up for classes like with Kaplan, their program has a few full length tests and they also have some more practice problems at their center and online for you to look at.
there are also a few free practice questions on this website:
http://www.testprepreview.com/oat_practice.htm
but i'm not sure if they're real questions or how helpful they are...
hope that helps! good luck