The last questions found in each section of the OG should be fairly representative of the harder to hardest questions most test-takers will see on the GMAT. After all, the OG questions were actual questions you'd encounter on past tests.

That being said, if you are scoring in the upper percentiles (85th and above), you may want to seek out harder problems to practice on. I'd recommend taking a few practice tests to gauge your level before you do so, however. Good luck!