My cousin has a OFNA MBX(i think, some type of OFNA buggy) but anyways when he starts it, it starts and then will rev really high and just kind of roll along the ground.

When he hits the throttle nothing happens.

Is this a problem with the clutch or something else?

it could be several things, but the clutch is where u would start. it only takes about 5 minutes to get the engine out, and clutch off for inspection, so check it. may just be really glazed up in there. before doing this - u can also do a quick check of the diffs by starting the car and applying the throttle. Look at the spur gear. does it spin fast, and the wheels do not. or does the spur gear and pinion gear stay almost stationary while the engine revs? this will help pinpoint if the problem is in the clutch or the vehicle itself.

So what would be the problem if the spur gear wasn't turning?

if the spur gear isnt turning, then that means that the problem is in the clutch

Ok, and if the spur gear is turning, then what?

Stripped diff component
Most like a broke gears or shear pin in one of the diffs

Hold three of the wheels from rolling and turn the last one; you shouldn't be able to turn it
If you can something is broke in one of the 3 diffs

Time to break down the buggy and inspect and replace the broken part or parts

Check the outdrive going into the diff. Occasionally, the setscrew will back out and the outdrive will just spin on the shaft.