Generally, the last 50 questions of any section in the OG are slightly more difficult than others. If you are finding the starting questions easy, why don't you directly proceed to the last 50 and try them.

Let me know how it goes!

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Good question, and I love the way you phrased it. "Is it sufficient?" - that really depends.

One of my favorite quotes is from legendary basketball coach John Wooden: "Never mistake activity for achievement". Just *doing* the OG12 isn't likely to be sufficient for most 700 aspirants. Like you eluded to, there aren't as many 80th-percentile-plus questions in the OG as people would think. Where OG12 is closer to sufficient is if you:

1) Take time to review the harder problems and find out why they're hard. For example there's a great problem in the Problem Solving section (it involves variables a and b; a remainder, and the number 96.12) that can give you a ton of insight into the test if you analyze what the author did. If you look at that problem and say "division", you're not getting full value out of it. If you look at it and say "they're making me reverse-engineer the concept of division by noting how the remainder relates to the decimal places", then you have more insight into hard problems - hard problems often make you reverse-engineer a particular step of a process that you've always done rote from beginning to end.

2) Review the problems that you got wrong *AND* those problems on which you guessed right (since you can't count on guessing right on test day) and find patterns in which types of problems give you trouble. Too often people just note their "percent correct" (e.g. "I did 85% correct in OG12. Does that mean I'll score 720?") and miss a valuable opportunity to learn from the problems they answered incorrectly, or "accidentally correctly".

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Now, in terms of difficulty level...I'd argue that the OG series probably isn't sufficient for most of us. If you think of a bell curve of GMAT scores, the vast majority are between, say, 460 and 650 or so. Which means that, because it's an adaptive test, most GMAT questions should be in that range, too. So the OG is going to be disproportionately weighted toward questions in the center of the curve.

If you need a challenge, you should really take the GMAT Prep tests multiple times, since the adaptive nature of those tests mean that the program has to feed you harder questions. If you're likely to be a high scorer, those GMAT Prep tests are even more essential than the OG because they'll make sure to challenge you under exam-like conditions.