| Abbreviation | Definition |
|---|---|
| ORSA | Operations Research Society of America |
| ORTA | Oak Ridges Trail Association |
| ORTA | Oak Ridges Trail Association [Ontario, Canada] |
| OSB | Order of Saint Benedict |
| OSC | Orlando Science Center |
| OSCC | Orchard Senior Citizen Center |
| OSCI | Open SystemC Initiative |
| OSD | Old Settlers Days |
| OSD | Open Source Developers |
| OSERA | Ohio Scientific Education and Research Association |
| OSF | Open Software Foundation |
| OSFB | Oregon School for the Blind |
| OSG | Objectivism Study Group |
| OSG | Oromia Support Group |
| OSI | Open Source Institute |
| OSP | Orphan Sponsorship Program |
| OSP | Orphan Support Program |
| OSPCA | Ontario Society for the Prevention of Cruelty to Animals |
| OSPL | open space and park land |
| OSRT | Ohio Special Response Team |