I would be very very wary of undertaking this path. Yes, it is possible to do, if the fellowship you seek has an unfilled slot. However, to the best info I can track down, you will never be Board eligible for your specialty.
{finding the answer to this question is like seeking a needle in a stack of needles}
Look carefully at the specialty board requirements. The ACGME ones will stipulate that you have completed an ACGME approved residency, likewise with the AOA specialty boards. How closely that Board interprets that language is anyone's guess.
At one time, I was seriously considering a pediatrics subspecialty & considered doing an AOA peds residency to be followed by an ACGME fellowship. I spoke with several DOs who practiced in the two areas of my interest and they all resoundingly cautioned me against pursuing a mixed route. The problem is not that it is specifically prohibited, but that yes/no falls into a grey area and no one is certain if it is OK. The danger being that you've done all of this training only to be told that no, neither the AOA nor the ACGME will agree to accredit your training.