A DO is a doctor of Osteopathy.
In the United States, both DOs an MDs are the only two degrees that qualifies a person as a licensed physician. Both complete 4 years of medical school and then residency training in hospitals and clinics within their specialty. Either degree can practice in any specialty – family medicine, oncology, neurology, general surgery, etc. In other countries, a DO is not trained as a physician and their scope of practice is thus limited.
DOs have historically emphasized a holistic approach to medicine with an emphasis on supporting the patients health. They receive over 400 hours of additional training beyond the standard medical school curriculum to refine their manual diagnostic skill and provide a hands-on treatment to support the healing process, known as osteopathic manipulative medicine (OMM).