My new (2014) Palomino's bottom and sides are 3/4" plywood, not OSB (that the lord for that) and the entire outer is skinned in pebble grained sheet aluminum.
The bottom of the bottom sits proud of the truck bed on formed steel extrusions running parallel to the camper with one at 90 degrees in the very front and one at the back. That adds rigidity and keeps the camper off the bed of the truck.
Obviously, much improved over the original units.
I'd say skin it (you can get the material from Palomino in Colon, Michigan or, coat the entire bottom with a truck bed liner material line Line-X or Rhino Liner, both rubber based and both water resistant. Think I'd add 1 x 6 Pressure Treated stringers to the bottom as well, to keep the bottom from direct contact with the truck bed.
As a side note, I do not have a bed mat in my truck, just the Line-X bed liner and I've never had a movement issue and I camp off road 99% of the time.
If your OSB starts out dry and you seal it dry, rot isn't an issue. Takes moisture and heat to start rot.
