Completely agree with Dopa. There isn't a field that is impossible for a DO to get into. It is all about how much work you put into it and how proactive you are about your future career. Some specialties are harder for DO's to get into, derm, ortho, and plastics come mind, but none of them are an absolute impossibility. Yes, there are plenty of programs that won't even consider a DO, but there isn't a field that 100% will not consider a DO, look at the NRMP PD data that was posted elsewhere on SDN.
In general, I would venture that many individuals in the DO realm had instability in their early academics, are changing careers, or have a difficult background that prevented them from obtaining the stats necessary for MD. Is there anything wrong with that? Not at all, but osteopathic schools are generally more forgiving of individuals with a shaky academic background who showed their ability later on down the line. If you are looking for somewhere where you aren't an MCAT score and a pair of GPA's, then the DO route is more suited for you. What you do once you get to a DO school determines where you go in the future as I'm sure plenty of people slack off in medical school but still turn out to be physicians.