Hi,
I would like to ask you a few questions before I can help you properly. Firstly, when you were writing the mock, did you feel your problem was related to knowledge or time? Secondly, are you answering all the questions in each section?
As for the OG, the difficulty level of the OG progresses as you progress through the questions in each section (i.e. toughest being at the end), so to state that the difficulty level of the OG isn't the same as GMATPrep is not an apples to apples comparison. The GMATPrep software will present you with questions based on your response to the previous question, so if you got a 700+ question right the next question would be tougher or in the same ballpark. To put it concisely, the difficulty level of the OG progresses in a linear fashion whereas the difficulty level of GMATPrep questions progress based on a proprietary algorithm.
The true purpose of the OG is to acquaint you with the kind of questions that come on the GMAT at varying levels of difficulty. It is not intended to be a comprehensive guide to cracking the exam. I would point you in the direction of the
MGMAT Strategy Guides if that's what you're looking for.
Hope this was helpful!
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