I was rather impressed with the ASRock X299E-ITX/ac's overclocking abilities. Considering the limited space available, I think this board does pretty well, although there are absolutely no hardware-based overclocking features, such as a POST display. Those dual-digit displays are immensely useful when overclocking any part of your system, and I really would have preferred it if one could have been added here, yet I do understand why it wasn't. Space is just at such a premium, as evidenced by the board's design itself and the add-in cards that fill out the board's native functionality. That said, I did manage to replicate my "standard" OC profile with my i9-7900X and did have luck in pushing both my i9-7960X and i9-7980XE, too, although these HCC chips are limited by the board's VRM when pushing the limits, and getting them to decent clocks, to where I wanted them, did require a lot of tweaking of the voltage options. Modest overclocking, however, is most definitely possible.
One of the things you'll want to do when you get this board installed, and if you plan to overclock, is to update to the most recent beta BIOS as fellow reviewer Steven Basiri noticed some options that might have been missing, and the most recent BIOS, v1.03, fixes that by adding in both an option for air or water cooling selectable in the BIOS (which adjusts power profiling to levels that suits each cooling type), and there is "Multi-core Enhancement", which boosts the Turbo profiling to levels that aren't present on the default-shipped BIOS.
The above screenshot simply shows the BIOS version so you know exactly what I am talking about, and which BIOS offers these options. Mr. Basiri (StevenB on our forums) deserves a high-five for getting ASRock to implement this! Thanks, Steven!
For those of you looking to just run stock clocks, the default BIOS is more than adequate and nicely tuned to keep power use and temperatures in check.