| Abbreviation | Definition |
|---|---|
| OAT | Oklahoma Antique Tractor Association |
| OBC | Old Baseball Cards |
| OCCB | Old Car Club of Bangladesh |
| OCCC | Oklahoma City Corvette Club |
| OCG | Original Card Game |
| OCGS | Olmsted County Genealogical Society |
| OCM | Ownership Collection Master |
| OCMA | Orange County Modelers' Association |
| OCMCC | Orchard County MotorCycle Club |
| OCSG | Open Canoe Sailing Group |
| OCW | Ozark Creative Writers |
| ODCA | Old Dragon Collectors Association |
| ODS | Our Dark Skies |
| OECC | Old English Car Club |
| OFA | Old Fortunates Association |
| OFC | Old Farts Club |
| OFC | Oregon Fishing Club |
| OFDA | Ohio Fast Draw Association |
| OFFC | Old Farts Flying Club |
| OFLAC | Oak Farm Leisure and Activities Club |