Unfortunately, even though DO's are able to apply to any of the ACGME residency programs, that is not reciprocated the other way for MD's to apply to any of the OCGME residencies. It's wrong and as a DO I think if I can apply for allopathic residencies the reverse should be true as well.
To be honest the entire match should be open to all applicants regardless of the letters behind your name. And they should combine them both into a single match