- 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
- Outsourcing Solutions, Inc.
- Ovation Research Group
- Overhead Door
- Overland Park Convention and Visitors Bureau
- Overland Park Regional Medical Center
- Overlook Medical Center
- Overnite Transportation Company
- Oviedo Medical Center
- Owen High School
- Owen Middle School
- Owens and Minor, Inc.
- Owens Community College
- Owens Corning
- Owensboro Health Regional Hospital
- Oxford Development Company
- Oxford Health Plans
- Ozanam Hall of Queens Nursing Home
- Ozark Foothills Regional Planning Commission
- Ozark Health