The real GMAT is adaptive - as you proceed through the test, the algorithm will discover your ability level and keep giving you questions that are challenging for you. The OG is not adaptive; it covers the entire difficulty spectrum, which means if you're well above average in ability, most of the OG will be easier than what you see on test day. There are other factors that have led the current GMAT to be somewhat more difficult than the OG; for one thing, the company developing the test changed a few years ago (after many OG questions were written), and it seems that newer questions are somehow a bit 'trickier' than older ones. And there's a second issue which I suspect has made the test increasingly difficult over the last few years. Test takers have been investing more and more time in preparation over the last decade. Because of this, they will answer their experimental questions (the questions which don't count, but which are inserted in your test to gauge their difficulty level for use as real questions in future tests) correctly more often than the less prepared test takers would have done 10 or 20 years ago. That means a question that might have been classified as a 700-level question ten years ago might only be classified as a 600 or 650-level question now.
That said, the OG is an essential resource during prep. It will give you a nearly comprehensive overview of the content of the test, and you'll get a very good feel for the 'style' of real GMAT questions. It's perfect to work from early on in your preparation. Just be aware that if you're a high-level test taker (in either Quant or Verbal), you'll see several questions on test day that are harder than most of the questions in the OG. You'll get a much better indication of what the actual test will be like from the GMATPrep software, as akhilsuhag pointed out above.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com
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