Well, I can say that i have literally kept track of all my OG answers, each time I have gone through them (Q 2.5x, V 1.5x). So, I know exactly my hit rate. I use an
error log/answer sheet I got off of this website... It is amazing! It has timers, % right, time, time per question and more. Search for it, it is worth it.
I take notes and review the questions I got wrong, plus during the practices I jot down my thoughts.
In the OG, you should (IMO) be above 85% in all sections.
Easy questions in OG are about first 2/5, next 2/5 are medum, and maybe last 1/5 are "hard." <--this is my opinion. (THis is for OG 11 where they are in order of difficulty.) However, the "hard" OG 11 questions are medium or easy in the GMATprep and GMAT. So, you should get 90% right in all OG 11 Q... others might feel differently, but that is my goal.