- 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
- OnEighty Advertising Agency
- OneLegacy
- OneSource Distributors, Inc.
- OneTouch Technologies Corporation
- Onslow Memorial Hospital
- Opelousas General Health System
- Open Door-BMH Health Center
- Open Works
- Operation Lifesaver Inc and National Support Center
- Operational Technologies Corporation
- Opnet
- Optical Gallery of Arizona
- Optical Laboratories Association
- Optima Products, Inc.
- Optimal Therapy Associates Services
- OptimuMedicine, LLC
- Options Unlimited, Inc.
- Optometrist
- Optometry Board
- OPUS Design Group
- Oquendo Velez S. Associados, Inc.
- Oracle
- Orange City Area Health System
- Orange Coast Association Of Realtors
- Orange County Association of Realtors
- Orange County Global Medical Center
- Orange County Government
- Orange County Health and Family Services Department
- Orange County Sheriff's Office
- Orange Ford Lincoln Mercury Inc.
- Orange Regional Medical Center