Hi 12921.
As you said, there aren't that many questions in the
OG. So, you're best off using the
OG for mixed practice after you have done topic-by-topic preparation and achieved high accuracy in topic-focused practice.
Regarding when to start doing timed practice, there are a few ways to look at the idea of timed practice. One is that, by developing skill, you'll naturally drive down your time per question, so that you won't necessarily have to do timed practice per se. Rather, you can become skilled enough that the time allotted per question by the test isn't really an issue. Also, as you prepare, you can look for ways to drive down the time you need per question. So, rather than simply cut yourself off at, say, two minutes per question, you could take all the time you need to answer questions but seek to start actively looking for ways to drive the amount of time down with the eventual goal of answering questions at test pace.
For more on how to progress from doing completely untimed practice to answering questions at test pace, see this post.
https://gmatclub.com/forum/mastering-th ... 58019.html