I think the answer depends in part on your goals. If you are shooting for a top score (700+), then the "Extra" MGMAT chapters have some very helpful material.

In any case, I would prioritize the OG ahead of these "Extra" chapters. It's strategically more important to solidify your ability to handle easy and medium difficulty questions than it is to learn how to handle the most difficult problems.

I would also recommend that you start to take practice exams - perhaps on a weekly basis. By taking practice exams on a regular basis, you'll get practice in integrating all of the necessary skills in an adaptive environment. You'll also have the opportunity to practice your pacing, which is very important on the GMAT.

Good luck!

Thanks spark.

Yes, I agree I should start practice exams but I didn't want to waste them until I had my bases covered.

I think I will do the non-extra chapters then do some OG questions then go back and do the extra chapters.