We have discussed this many times before. You CAN do an allopathic fellowship after osteopathic residency... BUT you cannot sit for the allopathic board exam because most of them clearly state that an ACGME residency is a prerequisite. I know a few attendings that were kinda screwed because of this. The AOA boards, because of lack of availability, will grant you "board certified" status after doing ACGME fellowship if you fill out their paperwork and sit for their test. That is why you can get osteopathic board certification in things like Toxicology, when there isn't a single Tox program in DO world. They review it and OK it.
There is a rumor, passed to me by the OB director at my hospital, that there are some changes happening with this situation. It seems that some of the governing boards are changing the rules about allowing us to sit for the exam. It makes perfect sense, but I don't know if it will come to pass. Rumors are just that. But I can't imagine programs being too pleased that their fellows, whom they deemed qualified to train, cannot sit for the exam. It is just silly outdated politics.
I know someone will bring up an example of a DO that worked around this... I don't know what to say other than they probably filed a special petition that few people know about. But as I said, I know gastro, peds EM, and repro guys, all fellow trained who could not sit for the allo boards. Some went the AOA route, others didn't care and just wanted the training. If you KNOW someone who accomplished this please post what they did... it would serve very useful to a lot of us. Please don't speculate or use hearsay, please try to use some sort of reference/evidence. Remember fellow is not the same as diplomate.
If you did ACGME residency and then you ACGME fellow there will be no problems. I am not sure what the deal is if you do ACGME residency and try for an AOA fellowship... albeit, it's probably a rare occurence.