- Oak HC/FT
- Oak Hill Capital
- Oak Hill Capital Partners
- Oak Hill Partners
- Oak Street Health
- Oakwest Healthcare Services Inc.
- OASIS-D
- Occupational and Environmental Medicine Journal
- Occupational Safety and Health Administration
- Ochsner Health System
- OCR
- OCS Inc.
- Odd Couples Housing
- Odyssey HealthCare
- Office of Inspector General
- Office of Legal Policy
- Office of Management and Budget
- Office of People with Development Disabilties
- Office of the Actuary
- Office of the Inspector General
