He's currently among the 16 fastest 400 meter runners on the planet. If the artificial limbs created any sort of disadvantage, even by the tiniest of margins, I don't think it would be possible for anyone with them to rank among the world's best; the level of competition at the very top is just too great. And what is the probability that the high tech carbon composite prosthetics (called "Cheetah Flex-Foot") produce identical results to that of a normal human foot and ankle? Again that seems virtually impossible. So if the artificial running legs aren't any worse than natural, and they aren't exactly the same, there is only one option left.
I guess there is no way to know with 100% certainty that the prosthetics make Oscar better; but the odds against it are infinitesimally small. It's the same way I feel about the one legged wrestler who won the NCAA championship last year.