O-1: Typically I would think of athletes/entertainers, in the US for competition/show/tour/etc... It may not apply to some young founder getting into YC who has no other track record of success; however, YC/Tech are politically connected so the mere fact one is accepted into YC, it might = extraordinary ability (in the eyes of the Gov. anyway)
J-1= I personally have never gone down this path in my legal career, so I am not 100% sure, but it is possible YC is part of an exchange program...if not, they should set up their own business education exchange and consider this especially for their fellowship program...boom legal hack (and except the fact I just gave it away, I could have applied to YC with the idea of hacking the legal system to get more visas for foreign founders joining US incubators...YC application hack)