- OSF Healthcare Saint Paul Medical Center
- OSF HealthCare Transitional Care Hospital
- OSF Heart of Mary Medical Center
- OSF Holy Family Medical Center
- OSF Little Company of Mary Medical Center
- OSF Medical Group - Ottawa
- OSF Sacred Heart Medical Center
- OSF Saint Anthony Medical Center
- OSF Saint Anthony Medical Center
- OSF Saint Clare Medical Center
- OSF Saint Elizabeth Medical Center
- OSF Saint Francis Medical Center
- OSF Saint James - John W. Albrecht Medical Center
- OSF Saint Joseph Medical Center
- OSF Saint Luke Medical Center
- OSF Saint Mary Medical Center
- OSF Saint Paul Medical Center
- OSF St. Mary Medical Center
- OSI
- Osicina Medicina Interm
- Osteogenesis Imperfecta Foundation
- Osteotech, Inc.
- Otero County
- Otero County
- Otsego County United Way, Inc.
- Otsuka Maryland Research Institute, Inc.
- Otterbein University
- Ottumwa Regional Health Center
- OU Edmond Medical Center
- OU Health- University of Oklahoma Medical Center
- OU Medical Center
- OU Medicine
- Ouachita County Medical Center
- Our Lady of Fatima Hospital
- Our Lady of Lourdes Medical Center
- Our Lady of Lourdes Regional Medical Center
- Our Lady of Lourdes Women's and Children's Hospital
- Our Lady of the Angels Hospital
- Our Lady of the Lake Ascension
- Our Lady of the Lake Regional Medical Center
- Our Lady's Center for Rehabilitation and Healthcare
- Our Place Drug and Alcohol Education Service Inc.
- OurLegacy
- Outagamie County
- Outback Steakhouse Home Office)
- Outcome
- Outdoor Power Equipment Aftermarket Association
- Outdoor Power Equipment Institute
- Outokumpu
- Outsourcing Management Group