I've been told that in New York State, at least, MDs and DOs can get MD license plates. According to the NYS DMV website there is no DO plate, just an MD one.
But why would you want either one? Not only do you make yourself a target of scams, but you'll be charged up the nose when you take your car to the shop.
It's crap. If you wanna park at the "DOCTORS VEHICLES ONLY" spots in and around the city, flip your ID onto your dashboard and hope for the best. It should work, although the metermaid who thinks she's T. J. Hooker may give you a ticket just for kicks.