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Nobody uses standard plywood anymore.
If the trusses or rafters are 24" apart or closer you should be fine.
But what I will do or what half the country is doing has no bearing on this case. The contract specifically called for plywood and a cheaper product was used. That makes for a very strong case in court. The very least you should get from him is a knee bending, begging apology and the cost difference between the two materials which would be anywhere from twelve to forty cents per sq ft of roof area.
But I would be demanding more than that, because you will be looking at those sags for many years, if he leaves it there.
There is a newer type of OSB Called Advantech that is stronger and has more moisture resistant glues that might be acceptable for roofing but that is still ourside the contract. I would measure carefully in my mind to decide whether he is merely careless and slovenly in his trade practices or whether he is a shyster who intentionally defrauded you to decide how to proceed.