The Official Guide for GMAT Review
The Official Guide for GMAT Review is a book published by The Graduate Management Admission Council (GMAC). The Official Guide (OG) contains all you need to know about the GMAT including descriptions of the test sections, why you need to take the test, and how business schools use GMAT scores. GMAT OG also contains hundreds of practice questions, but the explanations are not always as detailed as you might expect.
Q : Do I Have to Buy GMAT OG to Take the GMAT?
You do not have to buy this book in order to take the test, but we do recommend that you buy it. First, GMAT OG is the only source for retired questions that actually have been used on real GMAT exams. Second, it contains a full-length diagnostic test that can tell you what your score would be if you were to take the test right now. (Of course, to get an accurate score you must stick to timelines and not use a calculator or any other materials except scratch paper and a pencil.) Being printed, the diagnostic test included in the GMAT OG is not a CAT test (not adaptive), but GMAC states that results of this test will be a nearly perfect indicator of your GMAT score.
To be sure, OG alone is not sufficient to get you fully prepared for the GMAT Exam because theoretical material is very scarce, but it is an excellent source of practice questions. It's a good idea to learn the theory first, though, and only then start practicing with OG questions so that you do not waste them but use them to evaluate those weak spots you still have to improve.