The only rule I am aware of is that a DO cannot be licensed in 5 states (including PA) unless they do a one-year rotating internship prior to residency. After the internship, if you do a DO residency )as well as some MD residencies) you begin as a second year resident. I also heard somewhere else on this board that you can be licensed in those states without doing the rotating internship if you first get licensed somewhere else and then transfer. I should note that you have to do a rotating internship if you plan on doing a DO residency.