I went to a GI once (arguably more competitive than cards) who was a NYCOM alumnus (DO school). What's more important is doing awesome during your IM residency, meaning getting awesome LORs and doing research. I'd assume where you did your residency and how well you did in it determines if you get the fellowship moreso than your med school at that point. With that said, no, no one DO school prepares you for cards than the others.