| Abbreviation | Definition |
|---|---|
| ORFU | Ontario Rugby Football Union |
| ORSU | Oregon Rugby Sports Union |
| OSOA | Oil Sector Officers Association |
| OTR | Old Timers Reunion |
| OTU | Oceania Triathlon Union |
| OUAM | Order of United American Mechanics |
| OUE | Overseas Union Enterprise |
| OUE | Overseas Union Enterprises |
| OUG | Overseas Union Garden |
| OUP | Official Unionist Party |
| OUR | Orlando Union Rescue |
| OURS | One Union of Regional Staff |
| OUSD | Orinda Union School District |
| OUSU | Oklahoma University Students Union |
| OUSU | Oxford University Student Union |
| OWTU | Oilfield Workers Trade Union |
| OWTU | Oilfields Workers Trade Union |
| PACU | Piedmont Advantage Credit Union |
| PAGU | Pan American Gymnastics Union |
| PAKU | Pan American Karate Union |