Dr. O'Brien is Medical Director of Maternal-Fetal Medicine at South Shore Health. 

As a Maternal-Fetal Medicine physician, I am dedicated to caring for women and families navigating high-risk and medically complex pregnancies with expertise, compassion, and individualized attention. 

I strive to integrate evidence-based medicine with a holistic approach that recognizes the physical, emotional, and social factors that shape each patient’s experience. 

I am also committed to advocating for equitable healthcare and improving access and outcomes for marginalized and underserved communities.

William Tollefsen, MD is the EMS and MIH Medical Director for South Shore Health. Dual-boarded in Emergency Medicine and EMS Medicine, Dr. Tollefsen works clinically as an Emergency Physician at South Shore Health in addition to his role as EMS Medical Director.

One of few physicians in the country to receive the prestigious "FAEMS" recognition, Dr. Tollefsen is also the Medical Director for OEMS Region IV, serving 61 cities and towns of the Metro-Boston EMS Council. As the current EMS Medical Director for eight municipal and three private ambulance services on the South Shore, Dr. Tollefsen interacts and provides oversight for over 400 Paramedics.

Dr. Tollefsen graduated from New York Medical College and completed residency training in Emergency Medicine at Harvard University, Brigham and Women's, and Massachusetts General Hospital. He completed a fellowship in Emergency Medical Services and Critical Care Transport at the University of Massachusetts, where he worked as an Emergency Physician and Flight Physician for UMASS LifeFlight. He completed an MBA at the Isenberg School of Management at UMASS Amherst, and holds a Master of Medical Science degree from the Boston University School of Medicine.

A former pre-hospital provider, he is actively involved in EMS system design, paramedic education and Critical Care Transport Systems. He works closely with multiple EMS, Fire and Law Enforcement agencies throughout Massachusetts.

His research interests include systems of care, resuscitation, airway management, and paramedic education.

My treatment philosophy is to provide high quality, compassionate, and patient centered care.

Throughout my career at BU Medical Center and South Shore Health, I have divided my time between healthcare administration and direct patient care. Administratively, I have focused on medical education, patient safety and quality improvement. However, my true passion for medicine is driven by the bonds I build with my patients and caring for them throughout life.