| Acronym | Meaning |
 | ONI | (U.S.) Office Of Naval Intelligence |
| ONS | Operational Needs Statement |
| ONRA | Office National D'?tudes Et De Recherches A?rospatiales (French |
| OO | Object Oriented - Over & Out |
| OOAD | Object Oriented Analysis And Design |
| OOCL | Orient Overseas Container Line |
| OODA | Observation |
| OOO | Out Of The Office |
| OOP | Object-Oriented Programming - Out Of Print |
| OOS | Occupational Overuse Syndrome - OneSAF Objective System - Open Operating System - Out Of Service |
| OOTW | Operations Other Than War |
| OP | Observation Point/Post |
| OP CIT | Opere Citato (Latin |
| OPA | Other Procurement |
| OPAC | Online Public Access Catalogue |
| OPCON | Operational Control |
| OPCW | Organization For The Prohibition Of Chemical Weapons |
| OPEC | Organization Of Petroleum Exporting Countries |
| OPFOR | Opposing Force |
| OPI | Office Of Primary Interest - Open Prepress Interface |
| OPINTEL | Operational Intelligence |
| OPLAN | Operation Plan (also OpPlan) |
| OPORD | Operation Order |
| OPP | Ontario Provincial Police - Operational Planning Process - Other People's Privates |
| OPS | Operations |
| OPSEC | Operations Security - Open Platform For Security |
| OPSI | Overwhelming Post Splenectomy Infection - UK Office Of Public Sector Information |
| OPTAR | Operational Target |
| OPV | Offshore Patrol Vessel |
| OR | Operating Room (surgery) - Operational/Operations Research -Oregon (postal Symbol) |
| OR | Oriya Language (ISO 639-1 Code) |
| ORAU | Oak Ridge Associated Universities |
| ORB | #NAME? |
| ORBAT | Order Of Battle |
| ORD | Office Of Research And Development - Operational Requirements Document |
| ORD | O'hare International Airport In Chicago |
| ORF | Operational Readiness Float |
| ORI | Oriya Language (ISO 639-2 Code) |
| ORISE | Oak Ridge Institute For Science And Education |
| ORM | Oromo Language (ISO 639-2 Code) |
| ORNL | Oak Ridge National Laboratory |
| ORR | Oak Ridge Reservation (U.S. DOE) - Office Of Rail Regulation (UK Government) - Office Of Refugee Resettlement - (CIA) Office Of Research And Reports - Operational Readiness Rate - Operational Readiness Review |
| ORSA | Oxacillin-resistant Staphylococcus Aureus - Operations Research Society Of America |
| OS | Osmium |
| OS | Ossetic Language (ISO 639-1 Code) |
| OSA | Order Of St. Augustine |
| OSCE | Organisation For Security And Cooperation In Europe |
| OSD | U.S. Office Of The Secretary Of Defense |
| OSDL | Open Source Development Labs |
| OSE | Own Separate Entrance |
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