If you are not yet at the scoring level that you want, then you are not done with the OG problems, even if you have already done them. You have not exhausted those problems until you are hitting the score that you want to get.
You don't learn a ton simply from doing new problems. You do learn a ton from reviewing those problems thoughtfully and extensively. You can learn about the kind of review to do in these articles:
http://www.beatthegmat.com/a/2009/10/09 ... ce-problemhttp://www.beatthegmat.com/mba/2010/04/ ... our-errorshttp://www.beatthegmat.com/mba/2010/01/ ... g-questionhttp://www.beatthegmat.com/mba/2010/03/ ... y-questionI would start by going back to higher-numbered OG problems that you've already done and doing extensive analyses. THEN, you can start to test yourself on new problems to see whether you've learned what you were trying to learn.
First, learn by analyzing past problems. Then, test yourself by doing new problems. The new problems then become past problems, and you analyze them exhaustively before you try to test yourself on other new problems.
The MGMAT CATs will be a good test, yes, as will the question banks (if you have access to those). You can also use GMAT Focus for quant (
www.gmatfocus.com). You can also buy older OG books (some questions will overlap with later books but some will be new).