Hey there,
The truth is, while all the questions in both the OG13 and the GMATPrep are from the same official source: GMAC, not all questions are created equal. For example, some students find certain questions in the GMATPrep 3 and 4 a lot tougher than the GMATPrep 1 and 2, but as others have mentioned, it's all relative. Really, the GMAT will be hard for EVERYONE, because it will adapt (like a virus...okay, that's a bit harsh) to your ability quickly.
So I think the best mindset for practice is to EXPECT it to be VERY hard, then SEEK OUT THE TOUGHEST QUESTIONS YOU CAN FIND. Especially in your weakest areas. Once you've completed the OG questions, you may want to supplement your GMATPreps with some of the MGMAT CATs (you get access to all 6 of them by purchasing any of their books). MGMAT's CATs have some of hardest Quant I've seen around.
Another great resource is the 700+ level questions from GMATClub. Students cull the toughest problems and post them for discussion. I'd especially focus on Word Problems involving Rates, Mixtures, Sets, Probability, Combinations, or Permutations. Toss in some Functions and Sequences problems and you'll really be pushing into "tough Quant" territory.
https://gmatclub.com/forum/gmat-ps-quest ... 27957.html
Enjoy!
