OSB = Optimal swelling board. It's cheaper than plywood and perfect if it is never going to take on any water. Did you ever see what happens to OSB sub flooring if a builder drags his rear end and does not dry in a new house quickly while building it? I've seen every T&G and butt joint swell to where they need sanded or hit with a grinder prior to floor coverings. I have also seen sheets of the stuff that swell up from 3/4" thick to over 1" from taking on some H2O.
Use exterior rated plywood. For a the minimal amount you'll need marine grade would be a wise choice ($70-$80 sheet).
