Unless really really really well sealed... I don't use OSB for anything in the PNW as it will get wet at some point and rot out way faster than plywood. But that's because it will start to rain in sept, and it won't be dry till July. We'll get 2 months where things finally dry out, but in those two months... it will rain. So mildew and mold are normal things and if I can spend the few extra bucks per panel, I go with Plywood.