Numbers and statistics are not really useful I feel in this matter. If you are a DO applying for cardiology fellowship you will certainly be one of a very few. My year of application, there were 706 fellowship positions offered through the match, 14 of those went to DOs. I have no idea how many applied (my point is to point out that you will be a minority, not to imply that the odds are stacked against you). If this is what you want to do with your life, then numbers dont matter. I got bitten by the cardiology bug as an MS I and I was fortunate enough to have a few DO cardiologists as mentors. Go for it. Work your ass off, get the best medicine residency that you can, do as much research in cardiology as you can, submit abstracts, and try to get published.