4 YRS and the OSB is fine on mine. Plywood is defintey better at 5 times the cost but not necessary. we run iceshacks with OSB walls out across northen ont lakes and then put back in the bush in the offseason until we drag them out again. They certainly don't last forever but you'll get 10 yrs plus out of those OSB walls if you paint them like I have on mine.
Now having said that I have my other one in a swamp as well. Built on blocks in footings made by cedar logs. I had no desire to ver haul replacement boards down there (long trip hand bombing material) so I did plywood on that one.