Even though it may be possible, most osteopathic IM residencies are not regarded as prestigious as allopathic university IM residencies where more research is oftentimes being done and there is a sometimes wider range of pathology. I suspect that you would have a very difficult time getting a cards or GI allopathic fellowship coming from an AOA residency, but you may be able to get something like ID, endocrine, or rheum. Connections mean a lot when it comes time to get fellowships, so if you have someone who is well known in the allopathic research world pushing for your application, you will have an easier time then if you have an osteopathic attending at a community hospital.