| OMRC | Operational Maintenance Requirements Catalog | In English | → Maintenance and logistics |
| OMRF | Orbiter Modification and Refurbishment Facility | In English | → Maintenance and logistics → Spaceflight engineering, astronautics |
| OMRF | Orbiter Maintenance and Refurbishment Facility | In English | → Maintenance and logistics → Spaceflight engineering, astronautics |
| OMRP | Operations and Maintenance Requirements Plan | In English | → Maintenance and logistics |
| OMRS | Operations and Maintenance Requirements and Specifications | In English | → Maintenance and logistics |
| OMRS | Operations and Maintenance Requirements System | In English | → Maintenance and logistics |
| OMRSCB | Operations and Maintenance Requirements and Control Board | In English | → Maintenance and logistics |
| OMRSD | Operations and Maintenance Requirements and Specifications Document | In English | → Maintenance and logistics |
| OMS | Operations Management System | In English | → Management |
| OMSP | Operational Maintenance Support Plan | In English | → Management |
| OMSP | Organización Mundial de la Salud Pública | In Spanish | → Organizations and associations → International relations → Public health and hygiene |
| OMT | Object-Modeling Technique | In English | → Software |
| ON | Österreichisches Normungsinstitut | In German | → Standardization |
| ONA | Ontspanning Na Arbeid | In Dutch | → Organizations and associations → Sport, physical exercises |
| ONB | Op Nije Begon'n | In Dutch | → Organizations and associations → Sport, physical exercises |
| ONC | Organisme National de Certification | In French | → Standardization |
| ONDO | Op Naar De Overwinning | In Dutch | → Organizations and associations → Sport, physical exercises |
| ONE | Organisation Naturiste Européenne (www.onenaturism.com) | In English | → Organizations and associations |
| ONEP | Office National de l'Eau Potable | In French | → Organizations and associations → Environmental technology, water supply, waste management |
| ONG | Organisation Non Gouvernementale = ONGs | Organizações Não Governamentais | In Portuguese = NGO | Non-Governmental Organization | In English = NGO | Nichtregierungsorganisation | In German = ANG | Aozadur Nann-Gouarnamantel | In Breton = NRO | Ne-Registara Organizo | In Esperanto = ONG | Organizzazione non governativa | In Italian = LSM | Lembaga Swadaya Masyarakat | In Indonesian = ONG | Organización non gobernamental | In Galician = GKE | Gobernuz Kanpoko Erakunde | In Basque = ONG | Organización no gubernamental | In Spanish = ΜΚΟ | Μη κυβερνητικές οργανώσεις | In Greek = NNO | Nevládní nezisková organizace | In Czech = ONG | Organització no governamental | In Catalan = ННО | Неурядові неприбуткові організації | In Ukrainian = ONG | Organização não governamental | In Portuguese = NGO | Niet-gouvernementele organisatie | In Dutch = NVO | Nevyriausybinė organizacija | In Lithuanian |
In French | → Organizations and associations |
| ONG | Organización non gobernamental = ONGs, NGO, ANG, NRO, LSM, GKE, ΜΚΟ, NNO, ННО, NVO (look above) |
In Galician | → Organizations and associations |
| ONG | Organização não governamental = ONGs, NGO, ANG, NRO, LSM, GKE, ΜΚΟ, NNO, ННО, NVO (look above) |
In Portuguese | → Organizations and associations |
| ONG | Organització no governamental = ONGs, NGO, ANG, NRO, LSM, GKE, ΜΚΟ, NNO, ННО, NVO (look above) |
In Catalan | → Organizations and associations |
| ONG | Organizzazione non governativa = ONGs, NGO, ANG, NRO, LSM, GKE, ΜΚΟ, NNO, ННО, NVO (look above) |
In Italian | → Organizations and associations |
| ONG | Organización no gubernamental = ONGs, NGO, ANG, NRO, LSM, GKE, ΜΚΟ, NNO, ННО, NVO (look above) |
In Spanish | → Organizations and associations |
| ONGs | Organizações Não Governamentais = ONG, NGO, ANG, NRO, LSM, GKE, ΜΚΟ, NNO, ННО, NVO (look above) |
In Portuguese | → Organizations and associations |
| ONI | Ontspanning Na Inspanning | In Dutch | → Organizations and associations → Sport, physical exercises |
| ONLUS | Organizzazione non lucrativa di utilità sociale | In Italian | → Organizations and associations |
| ONR | Oranje Nassau Roden | In Dutch | → Organizations and associations → Sport, physical exercises |
| ONRAS | Occasional Notes of the Royal Astronomical Society | In English | → Serial publications, periodicals → Astronomy |
| ONS | Oranje Nassau Sportvereeniging | In Dutch | → Organizations and associations → Sport, physical exercises |
| ONS | Oranje Nassau Sneek | In Dutch | → Organizations and associations → Sport, physical exercises |
| ONT | Opeinde Nijega en De Tike | In Dutch | → Organizations and associations → Sport, physical exercises |
| OntoUCP | Ontology based Unified Communication Protocol | In English | → Software → Networks, communication |
| OntoUCP | Ontology-based Unified Communication Protocol | In English | → Programming languages |
| ONV | Op Nieuw Verenigd | In Dutch | → Organizations and associations → Sport, physical exercises |
| OOMM | Organizational Operations and Maintenance Manual | In English | → Maintenance and logistics |
| OOPS | Fundamental'nye Osnovy Opticheskoi Pamiati i Sredy | In Russian | → Serial publications, periodicals → Mathematics and natural sciences |
| OP | Operating Procedure | In English | → Management |
| OP | Organisation de Producteur | In French | → Organizations and associations |
| OP&R | Operations Planning and Requirements | In English | → Management |
| OPA | Operations Planning Analysis | In English | → Management |
| OPA | Online Publishers Association | In English | → Organizations and associations → Graphic industries, printing, publishing |
| OPA | Ontario Psychological Association | In English | → Associations → Psychology |
| OPAC | Online Public Access Catalog | In English | → Libraries |
| OPAM | Opera di Promozione dell'Alfabetizzazione nel Mondo | In Italian | → Organizations and associations → Education |
| OPAS | Organização Pan-Americana da Saúde (www.paho.org) = OPS | Organización Panamericana de la Salud | In Spanish = PAHO | Pan American Health Organization | In English |
In Portuguese | → Organizations and associations → International relations → Medicine |
| OPC | Open Process Control | In English | → Management |
| OPDQ | Ordre Professionnel des Diététistes du Québec | In French | → Organizations and associations → Personal health and hygiene |
| OPEC | Organization of Petroleum Exporting Countries = OPEC | Organization of the Petroleum Exporting Countries | In English = OPEP | Organisation des Pays Exportateurs de Pétrole | In French = OPEL | Organizaĵo de Petrol-Eksportaj Landoj | In Esperanto = OPEP | Organização dos Países Exportadores de Petróleo | In Portuguese = OPEP | Organización de Países Esportadores de Petroleu | In Asturian = OPEP | Organización de Países Exportadores de Petróleo | In Spanish = OPEP | Organització de Països Exportadors de Petroli | In Catalan = ОПЕК | Организација на земји извознички на нафта и нафтени деривати | In Macedonian |
In English | → Organizations and associations → Mining → 665 |
| OPEC | Organization of the Petroleum Exporting Countries = OPEP, OPEL, ОПЕК (look above) |
In English | → Organizations and associations → Mining → 665 |
| OPECST | Office Parlementaire d'Évaluation des Choix Scientifiques et Technologiques | In French | → Research policy → Politics → Engineering, technology |
| OPEIU | Office and Professional Employees International Union | In English | → Organizations and associations → Labour, labour economics |
| OPEL | Organizaĵo de Petrol-Eksportaj Landoj = OPEC, OPEP, ОПЕК (look above) |
In Esperanto | → Organizations and associations → Mining → 665 |
| OPEN | Organisation de Producteurs d'Énergie Nucléaire | In French | → Organizations and associations → Nuclear technology |
| OPEP | Organização dos Países Exportadores de Petróleo = OPEC, OPEL, ОПЕК (look above) |
In Portuguese | → Organizations and associations → Mining → 665 |
| OPEP | Organización de Países Exportadores de Petróleo = OPEC, OPEL, ОПЕК (look above) |
In Spanish | → Organizations and associations → Mining → 665 |
| OPEP | Organización de Países Esportadores de Petroleu = OPEC, OPEL, ОПЕК (look above) |
In Asturian | → Organizations and associations → Mining → 665 |
| OPEP | Organització de Països Exportadors de Petroli = OPEC, OPEL, ОПЕК (look above) |
In Catalan | → Organizations and associations → Mining → 665 |
| ÖPG | Österreichische Physikalische Gesellschaft | In German | → Organizations and associations → Physics |
| OPI | Operations Procedure Instruction | In English | → Management |
| OPI | Ośrodek Przetwarzania Informacji (www.opi.org.pl) | In Polish | → Research policy |
| OPID | Observatório da Profissão de Informação-Documentação | In Portuguese | → Libraries |
| OPIDF | Operational Planning Identification File | In English | → Management |
| OPM | Operations Procedure Manual | In English | → Management |
| OPML | Outline Processor Markup Language | In English | → Programming languages |
| OPP | Organizacje Pożytku Publicznego (http://opp.ms.gov.pl) | In Polish | → Organizations and associations |
| OPPBTP | Organisme Professionnel de la Prévention du Bâtiment Travaux Publics | In French | → Organizations and associations → Civil engineering → Building |
| OPPBTP | Organisme Professionnel de Prévention du Bâtiment et des Travaux Publics (www.oppbtp.fr) | In French | → Organizations and associations → Labour, labour economics → Medicine → Civil engineering → Building |
| OPPS | Operations Planning and Project Support | In English | → Management |
| OPPTIC | Organisation de Professionnels pour la Promotion des TIC en région Languedoc-Roussillon | In French | → Networks, communication → Organizations and associations → Telecommunications |
| OPR | Office of Primary Responsibility | In English | → Management |
| OPR | Operations Planning Review | In English | → Management |
| OPS | Operational Sequence | In English | → Management |
| OPS | Organización Panamericana de la Salud (www.paho.org) = OPAS, PAHO (look above) |
In Spanish | → Organizations and associations → International relations → Medicine |
| OpSCN | Optical Sciences Center Newsletter | In English | → Serial publications, periodicals → Optics |
| OPSDB | Operations Data Base | In English | → Data |
| OPSDBA | Operations Data Base Administrator | In English | → Data → Management |
| OPSIS | Observation et Prospective sur la Société de l'Information et ses Services | In French | → Computer science → Telecommunications |
| OptCo | Optics Communications | In English | → Serial publications, periodicals → Optics → Telecommunications |
| OptEn | Optical Engineering | In English | → Serial publications, periodicals → Optics |
| OptFT | Optical Fiber Technology | In English | → Serial publications, periodicals → Optics → Telecommunications |
| OptL | Optics Letters | In English | → Serial publications, periodicals → Optics |
| OptLE | Optics and Lasers in Engineering | In English | → Serial publications, periodicals → Optics → Electromagnetism |
| OptLT | Optics Laser Technology | In English | → Serial publications, periodicals → Optics → Electromagnetism |
| OptMa | Optical Materials | In English | → Serial publications, periodicals → Optics → 620 |
| OptN | Optics News | In English | → Serial publications, periodicals → Optics |
| OptPE | Optical Physics and Engineering | In English | → Serial publications, periodicals → Optics |
| OptPN | Optics & Photonics News | In English | → Serial publications, periodicals → Optics → Electromagnetism |
| OptQE | Optical Quantum Electronics | In English | → Serial publications, periodicals → 530145 → Optics → Engineering, technology |
| OptRv | Optical Review | In English | → Serial publications, periodicals → Optics |
| OptSp | Optics and Spectroscopy | In English | → Serial publications, periodicals → Optics → Electromagnetism |
| OPWC | Organisation for the Prohibition of Chemical Weapons (www.opcw.org) = OIAC | Organisation pour l'Interdiction des Armes Chimiques | In French |
In English | → Organizations and associations → International relations → Military engineering |
| OQE | Optical and Quantum Electronics | In English | → Serial publications, periodicals → 530145 → Optics → Engineering, technology |
| OQLF | Office Québécois de la Langue Française (www.oqlf.gouv.qc.ca) | In French | → Dictionaries → Linguistics |
| ORA | Operational Requirements Analysis | In English | → Management |
| ORAM | Output Random Access Memory | In English | → Memories |
| ORAP | Organisation Associative du Parallélisme | In French | → Computer science → Organizations and associations |
| ORB | Operations Review Board | In English | → Management |
| ORB | Object Request Broker | In English | → Networks, communication |
| ORC | Optical Character Recognition = ROC | Reconnaissance Optique de Caractères | In French |
In English | → Writing systems → Computer-based techniques |
| ORCHIS | Oak Ridge Computerized Hierarchical Information System | In English | → Computer science → Knowledge management |
| ORD | Operations Requirements Document | In English | → Management |
| ORDI | Ontology Representation and Data Integration | In English | → Software |
| ORF | Observer Research Foundation | In English | → Foundations |
| ÖRG | Österreichische Röntgengesellschaft | In German | → Organizations and associations → Medicine |
| OrGeo | Organic Geochemistry | In English | → Serial publications, periodicals → Geochemistry |
| ORI | Operational Readiness Inspection | In English | → Management |
| ORIENT | Oriental Research Infrastructure to European Networks | In English | → Research policy → International relations |
| ORLA | Optimum Repair Level Analysis | In English | → Maintenance and logistics |
| OrLi | Origins of Life | In English | → Serial publications, periodicals → Biological sciences |
| OrNav | Ortung und Navigation | In German | → Serial publications, periodicals → Geodesy, cartography |
| ORPHEM | Organisation Régionale des Pharmaciens Hospitaliers de l'Est Méditerranéen | In French | → Organizations and associations → Pharmacology, toxicology |
| ORR | Operations Requirements Review | In English | → Management |
| ORR | Operational Readiness Review | In English | → Management |
| ORRCA | Ontario Research Centre for Computer Algebra | In English | → Computer-based techniques → Algebra |
| ORROE | Occasional Reports of the Royal Observatory Edinburgh | In English | → Serial publications, periodicals → Astronomy |
| ORSDI | Oak Ridge Selective Dissemination of Information | In English | → Computer science → Knowledge management |
| ORT | Operational Readiness Test | In English | → Management |
| ÖRV | Österreichischer Rodel-Verband (www.rodel-austria.at) | In German | → Organizations and associations |
| OS | Operating System | In English | → Operating systems |
| OS | Operating Software | In English | → Software |
| OS | Operazione Sviluppo | In Italian | → Organizations and associations → Development |
| OS3B | Open Software Blanc Bleu Belge | In French | → Organizations and associations → Software |
| OSA | Ordo Sancti Augustini | In Latin | → Organizations and associations → Christianity |
| OSA | Oklahoma Sociological Association | In English | → Associations → Sociology |
| OSAJ | Optical Society of America Journal | In English | → Serial publications, periodicals → Optics |
| OSAJI | Optical Society of America Journal - Optics and Image Science | In English | → Serial publications, periodicals → Optics |
| OSB | Ordo Sancti Benedicti | In Latin | → Organizations and associations → Christianity |
| OSC | Orthense Sport Club | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSC | Ookmeer Schinkelhaven Combinatie | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSC | Oosterhoutse Sport Club | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSC | Olstse Sport Club | In Dutch | → Organizations and associations → Sport, physical exercises |
| OsCar | Osteoarthritis and Cartilage | In English | → Serial publications, periodicals → Medicine |
| OSCF | Operations Support Computing Facility | In English | → Computer science |
| OSCI | Organisation des Opérateurs Spécialisés du Commerce International | In French | → Organizations and associations → Trade |
| OSCIP | Organização da Sociedade Civil de Interesse Público | In Portuguese | → Organizations and associations |
| OSCR | Orbiter Software Change Request | In English | → Software → Spaceflight engineering, astronautics |
| OSCRS | Orbital Spacecraft Consumables Resupply System | In English | → Maintenance and logistics → Spaceflight engineering, astronautics |
| OSDL | Open Source Development Lab | In English | → Software |
| OSDL | Open Source Development Labs | In English | → Software |
| OSDO | Ons Samenspel Doet Overwinnen | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSE | Open System Environment | In English | → Computer science |
| OSE | Oeuvre de Secours aux Enfants | In French | → Organizations and associations → Politics |
| OSF | Open Software Foundation | In English | → Foundations → Software |
| ÖSFK | Österreichisches Studienzentrum für Frieden und Konfliktlösung | In German | → Research institutes → Polemology |
| ÖSFO | Ökosoziales Forum Österreich | In German | → Organizations and associations → Social sciences |
| ÖSG | Österreichische Schmerzgesellschaft | In German | → Organizations and associations → Pathology |
| OSI | Onze Sport Inspireert | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSI | Österreichisches Ost- und Südosteuropa-Institut (www.osi.ac.at) | In German | → Research institutes → Area studies |
| OSI | (Österreichisches) Ost- und Südosteuropa-Institut | In German | → Research institutes → Area studies |
| OSI | Open Source Initiative | In English | → Software |
| OSID | Operating System Interface Definition | In English | → Operating systems → Human-computer interaction |
| OSIEK | Organiza Societo de Internaciaj Esperanto-Konferencoj | In Esperanto | → Organizations and associations → Congresses |
| OSIM | Open Source In Morocco | In English | → Organizations and associations → Software |
| OSJaJ | Oceanographical Society of Japan Journal | In English | → Serial publications, periodicals → Marine research, oceanography |
| OSM | Omni Sportvereniging Maarssenbroek | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSM | Operational Status Monitoring | In English | → Management |
| OSMP | Operational Support Maintenance Plan | In English | → Maintenance and logistics |
| OSO | Omroep Sport Organisatie | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSPF | Open Shortest Path First | In English | → Internet |
| OSPG | Österreichisch-Südpazifische Gesellschaft | In German | → Organizations and associations |
| OSR | Oberster Sanitätsrat | In German | → Organizations and associations → Public health and hygiene |
| OSRBD | Operational Software Reliability Benchmark Development | In English | → Software |
| OSS | Oefening Staalt Spieren | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSSA | Oxfordshire School Sports Association | In English | → Associations → Sport, physical exercises |
| OSTDS | Office of Space Tracking and Data Systems | In English | → Data → Spaceflight engineering, astronautics |
| OSTP | Office of Science and Technology Policy | In English | → Research policy → Engineering, technology |
| OSV | Oostzaanse Sport Vereniging | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSV | Overdinkelse Sport Vereniging | In Dutch | → Organizations and associations → Sport, physical exercises |
| OSVIC | Organismo Sardo di Volontariato Internazionale Cristiano | In Italian | → Organizations and associations → Christianity |
| OT | Ordo Teutonicus | In Latin | → Organizations and associations → Christianity |
| OTDA | Office of Tracking And Data Acquisition | In English | → Data |
| ÖTI | Österreichisches Textil-Forschungsinstitut | In German | → Research institutes → Textile industry |
| OTL | Ordnance Test Laboratory | In English | → Research institutes |
| OTL | Online Tamil Lexicon | In English | → Dictionaries → Linguistics |
| OTS | Originaltext-Service | In German | → Newspapers, journalism |
| OVB | Organisatie ter Verbetering van de Binnenvisserij | In Dutch | → Organizations and associations |
| OVC | Ommense Voetbal Club | In Dutch | → Organizations and associations → Football and rugby |
| OVC | Oosterbeekse Voetbal Club | In Dutch | → Organizations and associations → Football and rugby |
| OVC | Oijense Voetbal Club | In Dutch | → Organizations and associations → Football and rugby |
| OVCS | Ophovense Voetbal Club Sittard | In Dutch | → Organizations and associations → Football and rugby |
| OVH | Ontspannings Vereniging Hercules | In Dutch | → Organizations and associations → Sport, physical exercises |
| ÖVI | Österreichischer Verband der Immobilientreuhänder | In German | → Organizations and associations → Accommodation |
| OVI | Operational Validation Inspection | In English | → Management → Engineering, technology |
| ÖVK | Österreichischer Verein für Kraftfahrzeugtechnik (www.?vk.at) | In German | → Organizations and associations → Mechanical engineering |
| OVS | Observation of Variable Stars | In English | → Serial publications, periodicals → Astronomy |
| OVV | Oostvoornse Voetbal Vereniging | In Dutch | → Organizations and associations → Football and rugby |
| OVV | Oosteinder Voetbal Vereniging | In Dutch | → Organizations and associations → 7963 |
| OVVO | Op Volharding Volgt Overwinning | In Dutch | → Organizations and associations → Sport, physical exercises |
| ÖWiG | Österreischische Wissenschaftsgesellschaft (www.oewig.at) | In German | → Organizations and associations → Research policy |
| OWIOS | Over Winnen Is Ons Streven | In Dutch | → Organizations and associations → Sport, physical exercises |
| OWL | Ostwestfalen-Darts-Liga | In German | → Organizations and associations → Sport, physical exercises |
| OWL | Web Ontology Language | In English | → Data → Programming languages |
| OxfOO | Oxford University Observatory Observations | In English | → Serial publications, periodicals → Astronomy |
| OxM | Oxidation of Metals | In English | → Serial publications, periodicals → Chemistry → Metallurgy |
| OYCF | Overseas Young Chinese Forum | In English | → Organizations and associations |
| OZC | ODIS Zorgvlied Combinatie | In Dutch | → Organizations and associations → Sport, physical exercises |
| OZC | Ommer Zaterdag Club | In Dutch | → Organizations and associations → Sport, physical exercises |
| OZK | Oranje Zwart Kromhout | In Dutch | → Organizations and associations → Sport, physical exercises |