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.

I am originally from Cambridge, Massachusetts. I practiced in Quincy for 30 years before coming to South Shore Medical Center in 2002. I am experienced in abdominal surgery, trauma, and management of skin and soft tissue tumors. 

I grew up on the South Shore and attended Holy Cross for college where my concentration was pre-medical sciences and sleep research. During summers in college I worked as an EMT and following college I worked in a collaborative Brigham and Women’s Hospital/Harvard Medical School lab studying allergy and asthma. While at the University of Massachusetts Medical School and Lahey Clinic my focus was on primary care internal medicine. In 2006 I was the recipient of the “Patient Care Award” which is Lahey Clinic’s highest physician honor. I joined South Shore Medical Center in 2006.

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.

Being a physician is an honor and a privilege I have enjoyed since graduating from The Johns Hopkins University School of Medicine in 1979.  As my patients know I am interested in your life, your passions and the Orthopaedic issues that frustrate you and how I might be of assistance to you.  Surgery is not always a solution.

I take care of newborn infants and their great grandmothers.  In an era of sub-specialization, I am the "silver haired" General Orthopaedist but have maintained my surgical skills and remain confident in my abilities.  I have been fortunate to have treated multiple generations of my friends and neighbors here on the South Shore and work in a hospital committed to clinical excellence and compassionate care.

I am honored to treat and work with my patients. When I am not working, I am running marathons or trails, skiing, biking or cooking. Health and wellness are some of my passions.

As the Medical Director of Addiction Medicine at South Shore Health, I work to tackle the defining medical challenge of our time: Substance use and alcohol use disorder.

In addition to seeing adult patients at South Shore Hospital, I work with colleagues across South Shore Health to develop and implement a comprehensive addiction medicine treatment program. This program will connect patients with the resources they need to manage substance use disorder, no matter if they come to us through the Emergency Department, primary care, or another setting. Additionally, I serve as Medical Director for Spectrum Health System's opioid treatment program, ATS/detox, CSS, and therapeutic community in Weymouth.

Prior to my work at South Shore Health, I helped launch the Bridge Clinic at Boston Medical Center, which connected patients with treatment options for substance use disorder. I also served as Medical Director for Spectrum Health Systems’ Opioid Treatment Program in Weymouth.

In my spare time, I enjoy baseball, reading fiction, snowboarding, and cooking.