OSB can be used ONLY if you are protecting it from the elements (put some kind of siding and shingles on the roof), it is not meant for long-term exterior exposure (it'll go to crap on you in a couple of months sitting out in the weather). If you are not looking to do anything to it (i.e. prime/paint, put any kind of siding on it), then a regular treated plywood (usually a southern yellow pine is used) would work fine...marine grade plywood is VERY expensive. If you don't mind priming/painting then you could use an AC grade of plywood (either yellow pine or fir is fine) but make sure that it gets sealed all around with your primer (especially the edges) and paint. You could also use a 5/8" T1-11 fir which is often used for exterior use (again, be sure to prime/paint it or use an exterior stain to seal it). Cedar plywood does exist, but it's hard to come by and the cost would cause you to fall off your chair.