UO
- Hong Kong Express Airways
- Orthodox Union
- U of O (disambiguation)
- Ultima Online
- Underoath
- Unexploded ordnance
- Universal Orlando
- University of Okara
- University of Oradea
- University of Oregon
- University of Osnabrück
- University of Otago
- University of Ottawa
- University of Oxford
- Urate oxidase
- Urban Outfitters
- Urge Overkill