For the opted.org sample exam, I would definitely obtain more practice materials for the reading comprehension portion since there's only one sample passage given. Completing one sample RC passage will probably not accurately reflect your true score on the real OAT. The opted.org materials gives you a starting point in your preparation for the OAT.
If you need more practice problems, the practice DAT is a good resource. Both are written by the ADA, so they're almost identical. With the DAT there's no physics, though, and the "perceptual ability" section isn't on the OAT. Here's the address: http://ada.org/prof/ed/testing/dat/dat_test_sampleitems.pdf
Good luck!
I studied that practise DAT as well. It is IDENTICAL in both the natural science section and QR to what you will see on the real OAT. Good find, I forgot to post this up but it is certainly helpful to say the least.