| O | Oberst - Observer - Obstacle - Octal - Officer - Official - Oh - Operational - Operator - Oprah (magazine) - Outstanding -Oxygen | - | Astronomy |
| OA | Operational Analysis | - | Astronomy |
| OAC | On Approved Credit | - | Astronomy |
| OARSI | Osteoarthritis Research Society International | - | Astronomy |
| OAS | Organization Of American States | - | Astronomy |
| OASIS | Objects And Agents For Information Systems And Simulation - Organization For The Advancement Of Structured Information Standards | - | Astronomy |
| OASIS | Ontario Agencies Supporting Individuals With Special Needs | - | Astronomy |
| OASIS | Ontario Association Of Sewage Industry Services | - | Astronomy |
| OASP | Operational Analysis Support Paper/Plan | - | Astronomy |
| OAU | Organization For African Unity | - | Astronomy |
| OB | Obedience - Obligation - Obligatory - Obsolete - Obstetrics -Ocean Beach (California) - Off Break (cricket) - Old Bryanstonian - Order Of Battle - Outlet Box (electrical) -Overbought (stock Market) -Oversight Board | - | Astronomy |
| OB | Obiit (Latin | - | Astronomy |
| OBE | Officer Of The Order Of The British Empire | - | Astronomy |
| OBJ | Objective (military Parlance) | - | Astronomy |
| OBS | Observation/Observer - Obsolete - Obstacle -Off Balance Sheet - Optical Burst Switching (IEEE) - Organic Brain Syndrome - Organization(al) Breakdown Structure | - | Astronomy |
| OBSS | Off Board Sensor Systems - Ordo Byantinus Sancti Sepulchri (Byzantine Order Of The Holy Sepulchre) - Orbiter Boom Sensor System (space Shuttle) - Origins Billion Star Survey | - | Astronomy |
| OBX | Outer Banks (North Carolina) | - | Astronomy |
| OC | Observer/Controller | - | Astronomy |
| OC | Occitan Language (ISO 639-1 Code) | - | Astronomy |
| OCA | Observatoire De La C?te D'Azur - Offensive Counter-Air | - | Astronomy |
| OCD | Obsessive-Compulsive Disorder | - | Astronomy |
| OCFS | Oracle Cluster File System | - | Astronomy |
| OCFS2 | Oracle Cluster File System Release 2 | - | Astronomy |
| OCI | Occitan Language (ISO 639-2 Code) | - | Astronomy |
| OCIDS | Optical Combat Identification System | - | Astronomy |
| OCN | Oncology Certified Nurse | - | Astronomy |
| OCO | Oort Cloud Object | - | Astronomy |
| OCOKA | Observation | - | Astronomy |
| OCR | Optical Character Recognition | - | Astronomy |
| OCS | See Entry | - | Astronomy |
| OD | Old Dragon - Ordnance Datum (nautical Charts) -Overdose | - | Astronomy |
| ODAS | OCA-DLR Asteroid Survey | - | Astronomy |
| ODB | Operational Data Base -Oxford Dictionary Of Byzantium | - | Astronomy |
| ODBC | Open DataBase Connectivity | - | Astronomy |
| OEA | (U.S.) Office Of Economic Adjustment | - | Astronomy |
| OECD | Organization For Economic Co-operation And Development (international) | - | Astronomy |
| OED | Oxford English Dictionary | - | Astronomy |
| OEG | Operation Exposure Guidance | - | Astronomy |
| OEM | Original Equipment Manufacturer | - | Astronomy |
| OFA | Occipital Face Area | - | Astronomy |
| OFDA | U.S. AID?s Office Of Foreign Disaster Assistance | - | Astronomy |
| OGC | Open Geospatial Consortium | - | Astronomy |
| OGD | Other Government Department | - | Astronomy |
| OGO | Other Government Organization | - | Astronomy |
| OGX | OUARGLA -a Petrolium State In The South Of Algeria | - | Astronomy |
| OH | Ohio (postal Symbol) | - | Astronomy |
| OHC | OverHead-Cam Engine | - | Astronomy |
| OHHLA | Original Hip-Hop Lyrics Archive | - | Astronomy |
| OHIP | Ontario Health Insurance Plan | - | Astronomy |
| OHP | Observatoire De Haute-Provence | - | Astronomy |
| OHSS | Ovarian Hyperstimulation Syndrome | - | Astronomy |
| OIC | Officer In Charge | - | Astronomy |
| OIE | World Organisation For Animal Health (originally The Office International Des ?pizooties | - | Astronomy |
| OIF | Operation Iraqi Freedom | - | Astronomy |
| OIOC | Oriental & India Office Collections (in The British Library) | - | Astronomy |
| OJ | Orange Juice - O.J. Simpson | - | Astronomy |
| OJ | Anishinaabe Language (Ojibwa) (ISO 639-1 Code) | - | Astronomy |
| OJI | Anishinaabe Language (Ojibwa) (ISO 639-2 Code) | - | Astronomy |
| OJT | On-the-Job Training | - | Astronomy |
| OK | Oklahoma (postal Symbol) -oll Korrect (facetious Alteration Of All Correct | - | Astronomy |
| OLED | Organic Light-Emitting Diode | - | Astronomy |
| OLS | Operational Linescan System | - | Astronomy |
| OM | Oman (ISO 3166 Digram) | - | Astronomy |
| OM | Oromo Language (ISO 639-1 Code) | - | Astronomy |
| OMA | Open Mobile Alliance - Operations & Maintenance | - | Astronomy |
| OMB | U.S. Office Of Management And Budget - Ontario Municipal Board -Order Of Mapungubwe In Bronze | - | Astronomy |
| OMD | Orchestral Manoeuvres In The Dark (1980s Pop Group) | - | Astronomy |
| OME | Osaka Mercantile Exchange | - | Astronomy |
| OMG | Object Management Group - Oh | - | Astronomy |
| OMN | Oman (ISO 3166 Trigram) | - | Astronomy |
| OMP | Order Of Mapungubwe In Platinum | - | Astronomy |
| OMR | Omani Rial (ISO 4217 Currency Code) | - | Astronomy |
| OMS | Order Of Mapungubwe In Silver | - | Astronomy |
| OMT | Object Model Template | - | Astronomy |
| ON | Ontario (postal Symbol) | - | Astronomy |
| ONC | Orthopaedic Nurse Certified | - | Astronomy |
| ONCE | Organizaci›n Nacional De Ciegos Espa?oles (Spanish | - | Astronomy |
| ONI | (U.S.) Office Of Naval Intelligence | - | Astronomy |
| ONS | Operational Needs Statement | - | Astronomy |
| ONÂRA | Office National D'?tudes Et De Recherches A?rospatiales (French | - | Astronomy |
| OO | Object Oriented - Over & Out | - | Astronomy |
| OOAD | Object Oriented Analysis And Design | - | Astronomy |
| OOCL | Orient Overseas Container Line | - | Astronomy |
| OODA | Observation | - | Astronomy |
| OOO | Out Of The Office | - | Astronomy |
| OOP | Object-Oriented Programming - Out Of Print | - | Astronomy |
| OOS | Occupational Overuse Syndrome - OneSAF Objective System - Open Operating System - Out Of Service | - | Astronomy |
| OOTW | Operations Other Than War | - | Astronomy |
| OP | Observation Point/Post | - | Astronomy |
| OP CIT | Opere Citato (Latin | - | Astronomy |
| OPA | Other Procurement | - | Astronomy |
| OPAC | Online Public Access Catalogue | - | Astronomy |
| OPCON | Operational Control | - | Astronomy |
| OPCW | Organization For The Prohibition Of Chemical Weapons | - | Astronomy |
| OPEC | Organization Of Petroleum Exporting Countries | - | Astronomy |
| OPFOR | Opposing Force | - | Astronomy |
| OPI | Office Of Primary Interest - Open Prepress Interface | - | Astronomy |
| OPINTEL | Operational Intelligence | - | Astronomy |
| OPLAN | Operation Plan (also OpPlan) | - | Astronomy |
| OPORD | Operation Order | - | Astronomy |
