Politics and Business magazines
- Ontario Teachers' Pension Plan
- Onyx Acceptance Corporation
- Onyx Software Corporation
- Open Text Corporation
- Operation Smile, Inc.
- Opinion Research Corporation
- Oppenheimer Wolff & Donnelly LLP
- Opsware Inc.
- Option Care Inc.
- Optische Werke G. Rodenstock
- Opus Group
- Oracle Systems Corporation
- Orange Glo International
- Orange S.A.
- Orascom Construction Industries S.A.E.
- OraSure Technologies, Inc.
- Orbital Sciences Corporation
- Orbitz, Inc.
- Orbotech Ltd.
- Orchard Supply Hardware Stores Corporation