Accuracy deteriorates as you go on solving the problems. The questions in the OG are comprised of 3 levels of difficulty with almost same number of questions in each level. Try learning a lot from mistakes. How to avoid the mistakes whatever you have done solving last time. Analyze more, solve less. Read posts by experts on how to go ahead with analysis, how much time to spend on solving and analyzing. Repeat the medium/hard problems again after a week or 10 days. We won't be remembering all the questions in mind. Try to download the OG error log available in the below link and the articles as well. It has topic/difficulty wise split up of OG questions.

https://www.beatthegmat.com/mba/gmat-error-log

OG Verbal is similar to OG 12 with additional 300 questions. Same 3 level difficultly in each section. If you have exhausted all your resources go for it.

Hey! thanks for the quick reply.
i realized that solving too many problems at a stretch has affected my accuracy. i'm mostly messing up the medium/hard questions with accuracies of 67-75 in them. i guess i'll go through concepts and then as you suggested, will get back to the difficult questions after 10-15 days. i am already using the error log for OG practice. it helps a lot.

i plan to take up the verbal and quant review books towards the end. i want to perfect all concepts and practice additional problems before i solve those books. i think jumping onto them right now will be a waste of official gmat material.