A D.O. is a Doctor of Osteopathy. D.O.s complete training that is equivalent to that of medical doctors, with additional training in manipulation. Osteopathic medicine is based on the theory that the body relies on proper structure and function to operate correctly and eliminate disease or injury. Doctors of Osteopathy emphasize the removal of internal or external abnormalities that threaten the body's natural systems. Their methods include manipulation as well as physical, medicinal, and surgical treatments.