Sheeting on the roof does two things. First, it gives you sheer strength. Second, it holds the nails used to attach the shingles. For sheer strength, OSB and Plywood are equal. For holding nails, OSB is significantly better. You have to by a high quality plywood to equal the low quality OSB sold at Home Depot to be equal, but if you are going to pay more for plywood, then Zip System OSB is going to be a much better choice. Nothing is better then Zip System!!!!
The problem with plywood is the voids and poor quality of each layer of wood. When you go into an attic an look at the sheeting from the underside, it's obvious how bad the plywood is by how much of it is blown out by the nails for the shingles. OSB is perfect around every single nail. And if you ever replace those shingle and nail over plywood a second time, expect the roof to fail a lot faster then the first time because of how damaged the plywood is.
Here in East Texas, 7/16 OSB is selling for $8 a sheet. It was $14 over the winter and sometimes it will get down to $6 a sheet. Half inch construction grade plywood is $18 right now and it rarely changes in price.
Leaving either outside and exposed to the elements will result in warping, delamination and failure. I'm not sure why anybody would be surprised at this. It's not designed for that.
