| Abbreviation | Definition |
|---|---|
| OPSA | Omaha Pet Sitters Association |
| OPSA | Overseas Plastic Surgery Appeal |
| OPSF | Occupational Physicians Scholarship Fund |
| OPSF | Olathe Public Schools Foundation |
| OPSG | OsteoPorosis Support Group |
| OPSI | Ohio Patient Safety Institute |
| OPSO | Older People Speak Out |
| OPTN | Organ Procurement and Transplantation Network |
| OPU | Oregon Parents United |
| OPVA | Oregon Paralyzed Veterans of America |
| OPX | Open Philanthropy Exchange |
| ORA | Ontario Rivers Alliance |
| ORC | Ocean Resource Center |
| ORFE | Oak Ridges Friends of the Environment |
| ORGASM | Official Royal Greenwood Association for the Stalkers of Myspace |
| ORH | Organization of Residential Homes, Inc. |
| ORIAS | Oak Ridges Institute for Applied Sustainability |
| ORIF | Orthopaedic Research and Innovation Foundation |
| ORMLT | Oak Ridges Moraine Land Trust |
| ORRF | Operational Risk Research Foundation |