While the degrees allow you to practice similarly, DO philosophy is something different. Having worked with both DOs and MDs in the same setting I can tell you there is a subtle difference in the way that DOs approach patients and the differential. And I'm not even talking about OMT here.
DO school is NOT just MD school with some OMT. There isn't quite as much emphasis on genetics, embryo, molecular bio and a few other basic science courses. I'm by no means saying that we don't learn them as well or as in depth, but there is a definite difference in time spent in those subjects. At my school we spend more time learning about the physical exam and how to properly examine patients than at most MD schools I'm sure. For me it works out great because I don't really care about research and knowing the names of the carriers of some crazy protein. Just give me the basic idea and let's move on. I want to learn how to treat patients. It's worked out well for me so far too.
I've taken both COMLEX and the USMLE and I can tell you that they are two different beasts of a test. While they both test the same concepts, the embryo, genetics and biochem on the USMLE was a nightmare for me to study for. If I had majored in basic science in college it might have been different, but I hated every minute of biochem so it was my worst subject. And genetics just sucks.
All of the OMT, cranial and chapman's points arguments aside, don't just go to DO school to be a doctor. Or because you couldn't get in to MD school. There is a distinct philosophy associated with osteopathic medicine that we need to keep around. We aren't just "MDs who can manipulate." There's the whole "wholistic", "patient centered" treatment approach that DOs claim to have, but any physician MD or DO who is a good physician will approach the patient's this way. We just are taught that way in school. I think learning OMT and having to practice on each other every week really lends itself to a better understanding of how to approach patients and read their body language. Are they in pain? Are they guarding? If I press on the abdomen do they cringe? What if I press harder? We learn so much about how to use our hands to better guide our senses which helps us form our impression of the patient and just how sick they are in our minds. Anyone can go up to a cholecystitis or an appendicitis and push on their belly, but if a light tough will do, why push as hard as you can?
Just my $0.02. Some late night ramblings while on surgery...