- Occupational Safety and Health Administration
- Ocean Healthcare
- Ocean Healthcare Group
- Ocean Healthcare Network
- Ochsner Clinic Foundation
- Ochsner Health System
- OCR
- Odd Fellow Home
- Odin Health & Rehab Center
- Office of Inspector General
- Office of the Actuary
- Office of the Actuary at the Centers for Medicare & Medicaid Services
- Ohana Pacific Health
- Ohio Health Care Association
- Ohio Living
- Ohio Long-Term Care Research Project
- Ohio Senior Living
- Ohio's Hospice
- Ohman Family Living
- OIG