What RAM does this MOBO actually support?

Ddr4 standard base speeds are 2133 and 2400, depending on the cpu. If you stick any speed other than those in the board, bios defaults will automatically run them at base speed. With Intel came XMP (sorta adopted by Amd too) which is eXtreme Memory Profile. On the ram itself is a seperate chip, houses the controller and its own bios (for lack of better terminology) which will include timing, speed and voltage variables as set by jadec tables you can see in cpu-z. So if you have 3200, it'll base at 2133, xmp1 will be 3200 (with correct timings etc), xmp2 will be 3000, xmp3 might be 2666 etc. All of these xmp settings are considered OC as they are beyond base speeds. Most all B & H model mobo's do not support any kind of OC other than a little BCLK adjustment, so don't support xmp profiles. Part of what happens with xmp also includes the memory controller in the cpu. It has limits. By overclocking the cpu you also bump up the ability of the MC, so is more likely to be compatible with the faster ram.

So really when you see stats of 2666(oc), 3200(oc) etc it's just meaning that the ram is set at an overclocked value (xmp) and it's entirely possible you might need some OC on the cpu to get that specific speed stable. Really only applies to Z rated boards for Intel and the B350 / X370 amd boards as the A320 doesn't support AM4 OC.

There's always the possibility of OC in ram, if you have 2400, it's possible to get it to run at 2666 with a manual OC, but you'll have to then find your own timings and voltage. If either is off a little the ram will be actually slower at 2666 than 2400 (you'll see this especially at boot), might not be stable, but mostly will be of no real help whatsoever. This all depends on the ram itself, of course, some ram takes OC better than others, some you might not be able to OC at all. Basically ram is best left at its best set speeds as you'll in no way be able to tell the difference, 1Hz = 1 operation per second, 2400MHz is 2.4Billion operations per second. Telling the difference between 2.4Billion and 2.66Billion operations per second is humanly impossible.
So run with a good xmp, and let the pc figure out whats best.