Hi rishchhabra,
When you take the Official GMAT, you'll be tested on many of the same concepts that you've seen while working through the
OG. However, you will NOT see any questions that are "copies" of questions from those study materials. The GMAT is a consistent, predictable Exam though - so if you've studied properly (and have a decent memory), then everything that you see on Test Day should remind you of questions that you've already answered (and concepts that you've already learned).
Focusing on 'tough' questions is probably NOT a good use of your time. However, before I can offer you the specific advice that you’re looking for, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied?
2) What materials have you used?
3) How have you scored on each of your CATs (including the Quant and Verbal Scaled Scores)?
Goals:
4) What is your goal score?
5) When are you planning to apply to Business School?
6) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich