i would not use the osb if your underside of the dwelling is going to be exposed. if you do i would seal the sheets with a heavy duty exterior primer pre-installation. once yer sub floor rots yer kinda screwed. you could put a 1/2'' treated ply and then go over that with a t&g osb . it will take more time but make sure you glue your sub floor down and nail it with ring shank galv. nails and deck screw the perimeter hit the "butt joints with screws 4-6" o.c. squeaky floors suck
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