| Abbreviation | Definition |
|---|---|
| OMC | Otara Millionnaire's Club |
| OMC | Owyhee Motorcycle Club |
| OMCH | Oude Motoren Club Herselt |
| OMSC | Oshawa Motor Sport Club |
| ONEC | Ottawa New Edinburg Club |
| ONEC | Ottawa New Edinburgh Club |
| OOAC | OFW OSCAR Alumni Club |
| OOCC | Oamaru Ordinary Cycle Club |
| OOCC | Oceanside Outrigger Canoe Club |
| OOCC | Ogasawara Outrigger Canoe Club |
| OOFC | Old Oundelian Football Club |
| OOWC | Oshawa Olympic Wrestling Club |
| OPC | Oklahoma Premier Clubs |
| OPC | Overseas Press Club |
| OPCC | Oak Park Country Club |
| OPCC | Orchard Park Country Club |
| OPCOM | Open Community |
| OPFC | Old Parmiterians Football Club |
| OPFC | Old Pauline Football Club |
| OPLC | Old Pueblo Lapidary Club |