Ask him to provide evidence that OSB is better. It certainly is cheaper, and let me assure you you won't be the one pocketing the savings. Plywood has some drawbacks vs. OSB, but if OSB is used in an area where wet service conditions exist (ie roof, exterior sheating, kitchen or bathroom floors) it will swell and fail time and time again. In fact there was a major settlement in 1996 (I think) from one of the major producers of OSB regarding exterior use failures. Now under purely dry service conditions, OSB performs as well as plywood structurally. If you allow the CONtractor to substitute OSB, the savings should be significant, so make sure and ask for a credit.