Yeah, we've been dealing with some leaking issues with our barn roof here, all around the bottom edges have gotten rotten from water getting in under the shingles. Would be really tempting to just rip out all the OSB and slap a metal roof on it. Might just opt to go with that Marine grade plywood to replace that bottom run of rotten OSB.The only problem with marine plywood is the cost.
Bad all the way around. 24 years ago we had a house rebuilt by a father/son pair. Super efficient. When we talked about roofs they said "drive by the condos over on ......" in the morning. They had worked on those buildings when working for a construction company. They would not use OSB as roof sheathing and I could see why. The asphalt shingle roofs on those fairly new condos were rippled. Exterior plywood was what they used, no ripples.
That said, the rebuilt part of the current house has 8" Structural Insulated Panels (SIPs, rigid foam sandwiched between 2 sheets of OSB) on the roof but instead of shingles the SIPs are covered with standing seam metal (we could have gone with asphalt shingles but I don't know if the roof would fair any better than those on the condos). And there is no attic space, the first layer on the rafters is 1" half lap pine (the ceiling) then the SIPS. And the exterior walls are 4" SIPS attached to the outside of the post and beam frame. The carpenter put on (new type) "tar paper" rather than the "housewrap" stuff because he said he has seen rotted sheathing in walls of houses not even 15 years old when covered with "housewrap" but much older houses with "tar paper" have always been fine. And over the "tar paper" he put vertical spacers and a "rain curtain" so that any humidity that might get behind the siding will condense and drain out rather than soak into the back of the siding. Besides rotting the siding out from the back, the moisture collecting in the wood will greatly decrease the "lifespan" of the paint as it "pops off" the wet wood.