Official questions will be more realistic than any others you can find. The difference is especially noticeable in Verbal, less so in Quant if you work from high-quality third party material.
The
OG will not feel quite as challenging as the real test, if you're already in a high scoring range. The
OG covers all difficulty levels, so a considerable majority of
OG questions will be below the level of a 700-level test taker. The
OG also still contains a lot of older questions, questions from back when the GMAT was more straightforward (from before ACT started developing the test), and some of them feel rather dated now.
You'll get a fairer impression of what the real test is like by taking a GMATPrep test. If your hit rate is high on questions even at the end of the
OG, it's extremely likely you'll score very well both on a GMATPrep and on the real test. But the test will definitely feel harder than the book. For further practice, the single best resource, if you've exhausted the books, is likely the official add-on question pack. Those questions are more contemporary than the
OG ones, and while the pack isn't specifically for higher level test takers, it does contain several challenging questions. I'd not recommend other sources for Verbal, but for Quant there are some good third-party question sets.
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