There are a few different ways to overclock on the X99A Tomahawk. You can go into the Command Center software and select one of the Game Boost modes.
You can go into the MSI Gaming App and select the OC Mode. Be sure that you have your CPU is selected. This will give you a small overclock.
Finally you can go into the BIOS and manually overclock. Moving the CPU voltage up to 1.35V and setting our multiplier to 43 we were able to achieve a stable overclock of 4.3 GHz. Now this is where we hit our thermal limit. If we have better cooling I know we could have gone higher.