You did some OG questions, then studied our book, then did more OG questions. Were the earlier set of questions lower in number? And the later set higher in number? The questions get more difficult as you work through the section in OG - so if that's what you did, then of course you wouldn't have done as well. You were trying harder questions. :)
The next step is to go back and figure out why you missed the questions that you missed, and why you spent extra time on the questions on which you spent extra time. (Did you time yourself? If not, time yourself in future.)
Actually, I also just realized: did you time yourself on the first set (before using our book)? On the second? Perhaps you took longer on the first set, then discovered that you're supposed to average only about 1m20s on SC, so then you tried to speed up - and that also would have led to a performance drop. Unlimited time is VERY different than having a time limit.
These two articles can help you to analyze individual problems and figure out what went wrong:
http://www.manhattangmat.com/articles/a ... roblem.cfmhttp://www.manhattangmat.com/articles/GMATprep-SC.cfmOnce you know what went wrong, that will tell you what to do next. Perhaps there are some rules that you don't know well enough after all. Perhaps you do know the rules but you fell into a trap or traps (the harder the questions get, the more likely people are to fall into traps based on what sounds good or sounds bad - your ear may be right on easier questions but it will be wrong on harder questions). Etc.
Next, you also want to make sure that your process is effective and efficient:
http://www.manhattangmat.com/blog/index ... n-problem/and this one for harder questions:
http://www.manhattangmat.com/blog/index ... orrection/Dig into all of that and let me know what you find.