Advantech OSB will outlast conventional plywood by a mile in the rain. I exposed it, flat (the worst way possible) as subfloor, to rains for 1+ year while building my house with only maybe 1mm of fiber degrading on the surface a few places. Many modern OSBs use hydrophobic glues impermeable to water.

Maybe but it's basically made of splinters. Not something you want in a thing you have to touch!

That's what I was thinking. Plywood is a horrible choice.