It depends on the school you go to, I'm sure, but many are taught more than just OMM. The actual class is called OPP in our case-- Osteopathic Principles and Practices. Part of it is learning about the history, philosophy, and art of osteopathic medicine that encompasses mind, body and spirit. There is a special emphasis in a "hands-on" approach that follows more than just manipulation. You spend an especially large part of your time diagnosing disorders with your hands and understanding the way illness presents, and you don't just use those skills in OMM lab; you use them in other courses as well. You get an amazing look at anatomy, and I'll guarantee you that you know it better than most any allopathic graduate around because you'll be studying it every week for the first two years at a minimum.
In our case, and in others I would imagine, the osteopathic philosophy is brought into as many of our courses as is possible. You are are taught to look at things from a different perspective. Yes, some people think it's kind of "hokey" and ignore it, but it's wrong to ignore history. Allopathic medicine comes from allos, meaning opposite and path, meaning disease. The traditional approach was to look at the symptoms and treat them with something that did the opposite. Osteopathy was designed with something different in mind: treating the body as a whole unit, rather than just the symptoms. While it's true that more allopathic programs are using this approach now, it's wrong to simply ignore that osteopathy was designed on those principles and those principles are still the core of the practice. The fact is that allopathic schools are simply becoming more like osteopathic schools, except for the manipulation. Some of them are even looking into that now.
It's easy to say that there is no difference when you have never been a part of it. It's easy to say that you'll go to a DO school, but you'll never accept OMM. It's much harder to look objectively at a situation you've never really been in and see where the real differences lie. Yes, there are currently more differences than just OMM. With the way things are going, though, that might not always be the case...but that's the way it is now.