There is an official sample test at:
http://www.ada.org/~/media/OAT/Files/oat_sample_test.ashx
It's good free practice. General consensus is that the natural sciences are easier than the real exam, but the physics, reading and QR are good representation (IMO).
I would also recommend Topscore. It is $60 for 3 full length exams in a software that mimics the conditions of the OAT. Its reading, natural sciences and math are ON POINT of the difficulty and material you are likely to see on the exam. However, the physics on Top Score is WAY OFF as it is very intense in content and calculation and was NOTHING like the real OAT (thank God). I was scoring 260-280 on their practice physics, and got a 330 on the real one. Also their reading comprehension grading scale is off, as I was getting 34/40 right and it was giving me like 300-310. But other than that, it is the closest software I know compared to the real thing, especially for the bio/gen chem/orgo parts.