Nothing official, but the general gist is..
#1: you need a really, really good reason to transfer, i.e. something to do with your spouse, your child/ren, parents, immediate family, etc.
#2: you need a letter from the dean of the originating DO school saying that they're allowing you to leave in good standing from the school (something to that effect)
#3: you need a letter from the dean of the receiving DO school saying that they're willing to accept you in good standing.
Number 3 is easy to get. Number 2, however, is more difficult, especially without Number 1. Also, depending on the school, you might have to repeat first year (variable).
I knew someone at my school who got a 3rd year DO to 3rd year DO transfer, so it can be done (not easily, though).