| Abbreviation | Definition |
|---|---|
| ONA | Ontario Nurses' Association |
| ONA | Oregon Nurses Association |
| ONAY | Onayi |
| ONEA | Oregon Narcotics Enforcement Association |
| ONS | Oncology Nursing Society |
| OOA | Ohio Optometric Association |
| OOA | Ohio Osteopathic Association |
| OORTSF | Object-Oriented Real Time Software Foundation |
| OOUG | Ohio Oracle Users Group |
| OOUG | Ottawa Oracle User Group |
| OPA | Ohio Pawnbrokers Association |
| OPA | Ohio Psychological Association |
| OPA | Oil Producers Association |
| OPA | Oklahoma Psychological Association |
| OPA | Oregon Pilots Association |
| OPA | Oregon Psychological Association |
| OPC | Ontario Press Council |
| OPCEA | Ontario Pollution Control Equipment Association |
| OPCMIA | Operative Plasterers and Cement Masons International Association of the United States and Canada |
| OPCS | Organisation de la Patrouille Canadienne de Ski |