I'll give it a whirl.
MD is the traditional title of physicians and comes from the european system of education. When the first colleges opened in the US they offered 6 month training programs and offered the MD degree.
In the late 1800's most physicians did not go to a school, but instead interned with a doctor for 2-4 years before calling themselves doctors. At the time of the civil war, the surgeon general(an MD) was court marshalled for refusing to give calomel to wounded soldiers. Calomel is a mercury compound that is so deadly you can't even touch it with bare skin. At this time there were no antibiotics, so almost any surgery was usually fatal.
So this guy A. T. Still, who was fed up with medicine because of the loss of his wife and children to meningitis, started his own study of medicine. He later came up with his own philosophys and opened his own school. He hired MD's to teach the core classes but then added on his own teachings he called osteopathy. He new little of pathology or even surgery, but he was very interested in the relationship between anatomy and disease. He discouraged the use of drugs and hoped to use surgery only as the last resort.
Over the next 40 years great advances were made in medicine and many of these were due to the osteopathic medicine. Harvard tried to require written exams for graduation, but it was opposed since most of the students could not write. Osteopathic schools went to a 2 year program and MD schools followed. I don't want to go off on a rant here so I'll just say that medical science finally caught up with the practice of medicine. The changes in medicine eventually made the curriculum at MD and DO schools so similar that there was a brief attempt to merge the two proffesions. The AMA let DO's in Claifornia buy an MD degree for 65$.
When DO's refused to stop doing OMM the deal was off and the two remained separate but equal. There is no procedure or treatment that an MD can give that a DO cannot. SO what's the difference?
Well, in the many years that osteopathic medicine has been around they learned a few things about physical medicine. There is a great deal of good you can do with your own two hands. MD's can learn it in a physical medicine residency. When you graduate from an Osteopathic Med school you already know it. At or school some of the OMM teachers are MD's.
Some people will tell you that the education is different, or even less than, an MD's education. I can only say that my classmates passed the USMLE exam at a higher rate than the national average for all MD schools. And we had to learn OMM on top of all that the MD students did.
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Jim