Exterior grade plywood is some very durable stuff. Eventually if constantly exposed to the elements it will delaminate and rot, but it takes a number of years.
If going to the trouble of trying to better seal the plywood, I would think that a commercially available exterior wood sealer would serve you better than mineral oil. I have used a type on log houses, and it seems to last about three or four years on areas directly exposed to sun and rain full...
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