Dr O’Connell graduated from Harvard Medical School in 1982 and completed residency in Internal Medicine at Massachusetts General Hospital in 1985. He is an Assistant Professor of Medicine at Harvard Medical School.

In 1985, Dr. O'Connell was the founding physician of the Boston Health Care for the Homeless Program and has been caring fulltime for homeless persons since that time. He is currently President of the program, which serves over 12,000 homeless persons each year in two hospital-based clinics and over 75 shelters and soup kitchens throughout Boston.

In 1985 Dr. O’Connell and his colleagues opened the nation’s first medical respite program for homeless persons in a local shelter. This respite program has evolved in response to sea changes in the health care system and is now a 104-bed facility that provides 24-hour medical and nursing care to ill and injured homeless persons. Working with the MGH Laboratory of Computer Science, he implemented the nation’s first computerized medical record for a homeless program in 1995.

From 1989 until 1996, Dr. O'Connell served as the National Program Director of the Homeless Families Program of the Robert Wood Johnson Foundation and the U.S. Department of Housing and Urban Development. Dr. O’Connell is the editor of The Health Care of Homeless Persons: A Manual of Communicable Diseases and Common Problems in Shelters and on the Streets. His articles have appeared in NEJM, JAMA, Circulation, the American Journal of Public Health, and the Journal of Clinical Ethics.