Here's something I've posted before in response to a similar question:
To understand Osteopathic medicine you have to go back & see why Dr. Still (who was an MD) started osteopathic medicine. Let me give you a bit of overview and perspective, but please don't consider this to be comprehensive & completely explanatory. He was practicing medicine in the 1800s, when medicine had little to offer patients, surgical procedures were traumatic (and had high mortaility rates), and many of the "medications" were toxic (or useless).
From his experiences working with patients, Dr. Still stated that the body is capable of healing itself (not, mumbo-jumbo...keep in mind what medicine consisted of in his day and age) and he began to study the human body in more depth, especially the musculoskeletal system. He also worked to determine why the body succombed to disease. Again, this line of thinking & study was unusual for physicians of the 1800s. In his research, he did find some manipulative techniques that improved some conditions for his patients (the beginning of osteopathic manipulative medicine/techniques...which has been tremendously expanded in the meantime). He also developed the "philosophy" of osteopathic medicine. Again, keeping in mind how radical this was for the time period, he wrote of "treating the whole person, not just the disease," and he stated that "the goal is to find health, anybody can find disease."
Osteopathic medicine has a stated philosophy/principles. You'll find slightly different wording from place-to-place, but here are the key principles:
1. the body is a unit: mind, body, spirit
2. the body is capable of self-regulation, self-healing, and health maintenance
3. structure and function are reciprocally interrelated
4. rational therapy is based upon the understanding of the first 3 principles
Although I know of no "stated philosophy" of allopathic medicine, these concepts are not foreign to MDs.
So basically, in a time when physicians could often do nothing or try things that too often caused the patient more harm, Dr. Still was practicing quite differently.
Now, by today's standards, the difference & the philosophy (in practice) are not tremendously different between MD & DO, but there are some. Obviously, DOs learn OMM (which some use and others do not). Also, there is an emphasis on the principles of osteopathic medicine during the clinical classes. Again, these principles are not something that MDs would disagree with, but in many (not all) allopathic schools they are not going to be regularly emphasized.