you hit the nail on the head with OSB. houses built with it that get rained on and then covered up not dry is almost a guarantee for mold. cdx plywood is glued together with a water proof glue. it has a 20 year wet rating like bare, no paint on the outside of a building. I bought 7 sheets of 3/8 at Menards for a water damage mitigation project. it was sanded on one side and beautiful. the contractor I hired bought an additional 8 sheet from where I do not know but it looked like dog ****, I mean really, voids, layers folded over and knots everywhere. you just have to look around, be picky and hand select from the stack. while cdx isn't all that cheap, its your best bang for the buck.