Obviously, you're not going to see the same questions on the actual GMAT as the ones in the OG. Still, the OG is the best study material - period. Make sure that you're taking away general rules/tips/tricks/shortcuts/whatever from every question you do. Simply "doing" problems won't help, since you won't see the same problem again. You need to develop GMAT sense, a framework for approaching the test, and a repertoire of tricks that you can apply to a range of problems. I'd recommend going back to the OG and focusing on finding takeaways in every problem. If you truly understand the underpinnings of every problem in the OG, you WILL get a good score on the GMAT.