Chuck,
REI and GYN Oncology are very competitive and I don't believe very many D.O.'s have been excepted in to those fellowships. I don't believe that any D.O. residency's have a fellowship in either REI and GYN oncology. I think there are 2-3 D.O. fellowships in MFM. However, allopathic MFM fellowship would a lot either to get into than REI, GYN/ONC, or cardiology as a D.O. You have a long time to decide what you want to do. Do what you want. I am a fourth year at AZCOM and want to do MFM and a classmate is going for a fellowship in GYN oncology. Additionally, a fellowship in either GI or Cards whould be one to two years less time depending on the program.
Matt