- Oklahoma Society of Association Executives
- Oklahoma State Orthopaedic Society
- Oklahoma State University Medical Center
- Oklahoma Surgical Hospital
- Oklahoma Transplant Institute
- Oklahoma University Medical Center
- Olathe Medical Center
- Old Dominion Electric Cooperative
- Olean General Hospital
- Olinger Crown Hill Mortuary and Cemetery
- Olinger Mortuary and Cemetery
- Olive Garden
- Olive View-UCLA Medical Center
- Oliver Williams and Davis
- Ollie Steele Burden Manor
- Olmsted Medical Center
- Olympia Medical Center
- Omaha Public Power District
- Omaha Standard, Inc.
- OMARA Organization
- Omega Health Care
- OMMI - Means
- Omni Corpus Christi Hotel
- Omni Hotels
- Omni Indianapolis North Hotel,
- Omni Shoreham Hotel
- Omni Sonics
- Omnicare, Inc.
- Omnicomm Systems, Inc.
- Omniplex World Services
- Omniti
- Omnitourism
- OMNOVA Solutions, Inc.
- OMS Motion, Inc.
- On-Board Engineering
- OnBase, Hyland Software Inc.
- Oncology Nursing Society
- ONCOR International
- One Medical
- One n Ten
- One Touch Technologies Corporation
- One United Bank
- One World Thru Youth
- Oneida County Health Department
- Oneida County Tourism
- Oneida Health Hospital
- Oneida Teachers Association
- OneLegacy
- OneSource Distributors, Inc.
- OneTouch Technologies Corporation