Tyler, Rich is fine. I go by either.

When I originally planed the HT, before the house was built, I was worried about moisture. So I went with OSB rather then ply. Never considered MDF because …

OSB will hold up if it get wet (although the edges might swell if it stands in water) where MDF will fall apart. OSB is stronger and you can screw into it and it will hold. MDF is extremely dense. If you don’t drill a pilot hole a screw or nail would be hard to get in. Plus there is a high chance of the MDF cracking and/or lifting/flaking the surface around the screw. Even with a pilot hole a screw may come loose if there is enough vibration. There is also the issue of flex. OSB holds up way better if it’s going to be flexed. MDF will fall apart if flexed enough. … I’m pretty sure, although not 100%, that MDF is stiffer and therefore less desirable for this application. Basically, MDF has its applications. IMO this is not one of them.