From what I have seen, if you are applying for a residency program that starts as a PGY-2 (such as neuro, radiology, anesthesia, PM&R, etc), it shouldn't be any problem. You apply for osteopathic internship and the allopathic residency at the same time, since those residencies fill 2 years ahead of time (ie, if you want to go into neurology, you apply this year for a spot for the class starting in 2008), so by the time you are done with the internship you are ready to go into the advanced program. For programs like general surgery, internal medicine, EM, Peds, Ob/Gyn, there is no prelim/transitional intern year. You go straight into the categorical program when you finish med school. Most programs usually do not accept a prelim or transitional year in place of the categorical intern year. These programs only interview for a spot one year in advance (ie, you interview in EM now for a spot beginning in 2007). I've seen a few DOs do these types of programs. Basically, you have to apply for an osteopathic internship, do that year, and then apply again for a categorical spot the following year. Basically, you have to go through the whole interview process twice.