Search by letter: O
Search by letter
Search by letter: O
- Ongar railway station — former Central line terminus in Chipping Ongar Ongar railway station
- Õngu — village in Hiiu County, Estonia Õngu
- Onhaye — municipality in Namur province, Belgium Onhaye
- Oni: Japanese folkloric creatures and other uses of the name Oni
- Oni (Japanese folklore): appearance, history, and cultural roles Oni (folklore)
- Onifai (Oniài) — town and comune in Sardinia, Italy Onifai
- Oniferi — town and comune in Sardinia, Italy Oniferi
- Ōnin era (1467–1469) Ōnin
- Ōnin War (1467–1477) Ōnin War
- Onion — Allium cepa: overview, characteristics, uses and history Onion
- Onionhead (1958 film) Onionhead
- Onley, Virginia
- Online and offline: connected and disconnected states in computing Online and offline
- Online game
- Online Mendelian Inheritance in Man (OMIM): a concise reference for human genes and phenotypes Online Mendelian Inheritance in Man
- Online pharmacy: how it works, regulations, benefits, risks, and consumer guidance Online pharmacy
- Online shopping
- Only Connect
- Only Fools and Horses
- Only Girl (In the World) — song by Rihanna Only Girl (In the World)
- Only Human (Dina Carroll album) Only Human
- Onnens (Vaud) — municipality in Jura-Nord vaudois, Switzerland Onnens
- Ōnojō — city in Fukuoka Prefecture, Kyūshū, Japan Ōnojō
- Onomastics — the study of names, naming systems, and name history Onomastics
- Onomatopoeia (words that imitate or suggest sounds) Onomatopoeia
- Onon River (Mongolia and Russia) Onon (river)
- Onondaga County, New York
- Onryō: Vengeful Spirits of Japanese Folklore, Theater, and Film Onryō
- ONS coding system: hierarchical geographic codes for UK statistics ONS coding system
- Onsernone — municipality in Ticino, Switzerland Onsernone
- Onslow County, North Carolina
- Onslow, Iowa
- Onslow, Nova Scotia (Colchester County) Onslow, Nova Scotia
- Ontario (province of Canada) Ontario
- Ontario County, New York
- Ontario Highway 10 (King's Highway 10) Ontario Highway 10
- Ontario Highway 101 - remote east–west provincial route to Quebec Ontario Highway 101
- Ontario Highway 11
- Ontario Highway 118
- Ontario Provincial Highway 12 Ontario Highway 12
- Ontario Highway 144
- Ontario Highway 17 — Ontario's Longest Highway and Trans‑Canada Route Ontario Highway 17
- Ontario Highway 21 (Bluewater Highway) Ontario Highway 21
- Ontario Highway 23
- Ontario Highway 24
- Ontario Highway 26 (King's Highway 26) Ontario Highway 26
- Ontario Highway 28
- Ontario Highway 33 (King's Highway 33) — Loyalist Parkway Ontario Highway 33
- Ontario Highway 34
- Ontario Highway 4 (King's Highway 4) Ontario Highway 4
- Ontario Highway 400 (King's Highway 400) Ontario Highway 400
- Ontario Highway 401: major east–west corridor in southern Ontario Ontario Highway 401
- Ontario Highway 402 (King's Highway 402) Ontario Highway 402
- Ontario Highway 407 (407 ETR) — tolled express route north of Toronto Ontario Highway 407
- Ontario Highway 409
- Ontario Highway 410
- Ontario Highway 416 (Veterans Memorial Highway) Ontario Highway 416
- Ontario Highway 417: major east–west freeway serving Ottawa and eastern Ontario Ontario Highway 417
- Ontario Highway 5
- Ontario Highway 6
- Ontario Highway 62
- Ontario Highway 7: Route, History and Role in Ontario's Road Network Ontario Highway 7
- Ontario Highway 8
- Ontario Highway 89
- Ontario Highway 9
- Ontario Hockey League (OHL) Ontario Hockey League
- Ontario Lacus — a hydrocarbon lake on Titan Ontario Lacus
- Ontario Liberal Party
- Ontario New Democratic Party
- Ontario, California
- Ontario, Ohio: city profile and overview Ontario, Ohio
- Ontario, Oregon
- Ontology (philosophy): the study of being, existence, and reality Ontology
- Ontonagon County, Michigan: overview, geography, history, and features Ontonagon County, Michigan
- Onward
- Onward (2020 film) Onward (movie)
- Onyx: banded chalcedony used as gemstone and ornamental stone Onyx
- Onyx River — Antarctica’s longest seasonal meltwater stream Onyx River
- Oobi — minimalist children's puppet series Oobi
- Oodgeroo Noonuccal: Aboriginal poet, activist and cultural educator Oodgeroo Noonuccal
- Oodnadatta, South Australia
- Ooid: small concentrically layered carbonate grain Ooid
- Oologah, Oklahoma: small town, lake, and Will Rogers heritage Oologah, Oklahoma
- Oomycete (water mold): biology, life cycle, and impact Oomycete
- Oona Chaplin — Spanish-British actress and member of the Chaplin–O'Neill family Oona Chaplin
- Oona O'Neill (Lady Chaplin) Oona O'Neill
- Ooooooohhh... On the TLC Tip
- Oophorectomy
- Oops!... I Did It Again
- Oops!... I Did It Again (song)
- Oort cloud — the distant, icy reservoir of long‑period comets Oort cloud
- Oost Gelre
- Oostende (Ostend) — Belgian coastal city in West Flanders Oostende
- Oosterscheldekering — the Eastern Scheldt storm surge barrier Oosterscheldekering
- Oosterzele: municipality in East Flanders, Belgium Oosterzele
- Oostkamp, West Flanders Oostkamp
- Oostrozebeke — municipality in West Flanders, Belgium Oostrozebeke
- Ooststellingwerf — municipality in Friesland, Netherlands Ooststellingwerf
- Oostzaan — municipality in North Holland, Netherlands Oostzaan
- Ooty (Udhagamandalam): hill station in the Nilgiris Ooty
- Op art: Optical illusion and visual perception in modern art Op art
- Opa-locka, Florida: history, architecture, and community overview Opa-locka, Florida
- Opabinia — a strange Cambrian animal with five eyes Opabinia
- Opacity: optical, material, digital, and metaphorical meanings Opacity
- Opal: formation, properties, varieties and uses Opal
- Opcode: the basic code that tells a CPU what operation to perform Opcode
- OPEC — Organization of the Petroleum Exporting Countries OPEC
- Opel (Adam Opel AG): German automobile manufacturer Opel
- Opel Adam — compact city car by Opel (2012–2019) Opel Adam
- Opel Agila — compact city car produced 2000–2015 Opel Agila
- Opel Antara
- Opel Astra (compact car) Opel Astra
- Opel Cascada — compact four-seat convertible Opel Cascada
- Opel Combo
- Opel Corsa: compact hatchback produced since 1982 Opel Corsa
- Opel Insignia
- Opel Karl — compact city hatchback sold as Vauxhall Viva in the UK Opel Karl
- Opel Meriva: compact multi-purpose vehicle (2002–2017) Opel Meriva
- Opel Mokka (subcompact crossover SUV) Opel Mokka
- Opel Omega — executive car (1986–2003) Opel Omega
- Opel Signum
- Opel Sintra — large MPV (1996–1999) Opel Sintra
- Opel Vectra — mid-size family car (1988–2008) Opel Vectra
- Opel Vivaro (light commercial van) Opel Vivaro
- Opel Zafira — compact multi-purpose vehicle (MPV) Opel Zafira
- Opelika, Alabama — Overview, History, and Local Features Opelika, Alabama
- Opelousas, Louisiana: history, culture, and civic profile Opelousas, Louisiana
- Open All Hours
- Open-bolt firing mechanism Open bolt
- Open class (linguistics) Open class
- Open content: definition, licenses, history and practical uses Open content
- Open Database License (ODbL) Open Database License
- Open Door Policy (China, late 19th–early 20th century) Open Door Policy
- Open Handset Alliance
- Open license
- Open Road
- Open Season
- Open Season (1974 film) Open Season (1974 movie)
- Open Season (movie series)
- Open Season (2006 film) Open Season (movie)
- Open Season 2
- Open Season 3
- Open Shortest Path First (OSPF) — Link-State Interior Routing Protocol Open Shortest Path First
- Open Site — free online encyclopedia Open Site
- Open source software Open source
- Open star cluster
- The Open University (United Kingdom) Open University
- Open University of Israel
- Open-wheel car (single-seater racing car) Open wheel car
- Open world (video games) Open world
- Openair Frauenfeld: Switzerland's Largest Hip Hop Festival Openair Frauenfeld
- OpenBSD: a security-focused BSD operating system OpenBSD
- OpenDocument (ODF): an open file format for office documents OpenDocument
- OpenGL — Open Graphics Library OpenGL
- Opening act: role, types, history and significance in live performance Opening act
- Apache OpenOffice: free open-source office productivity suite OpenOffice
- OpenOffice Base: desktop database front-end and embedded engine OpenOffice Base
- OpenOffice Calc: Overview, features, compatibility and history OpenOffice Calc
- OpenOffice Draw — vector graphics and diagramming tool OpenOffice Draw
- OpenOffice Impress (presentation software) OpenOffice Impress
- OpenOffice Math — Formula editor for the OpenOffice office suite OpenOffice Math
- OpenOffice Writer — the word processor of Apache OpenOffice OpenOffice Writer
- OpenSolaris — open-source descendant of Solaris OpenSolaris
- OpenStep — NeXT's Portable Object-Oriented API and Operating Environment OpenStep
- OpenStreetMap — the collaborative open geographic database OpenStreetMap
- openSUSE — Community Linux distribution openSUSE
- OpenVMS: a resilient enterprise operating system OpenVMS
- OpenVPN: protocol, features, security, history and common uses OpenVPN
- OpenWrt: a customizable Linux-based firmware for network devices OpenWrt
- OpenZaurus — Debian-based Linux for Sharp Zaurus PDAs OpenZaurus
- Opera: the musical drama — history, structure, and performance Opera
- Opera (web browser)
- Opera buffa: the Italian comic opera of the 18th century Opera buffa
- Opéra de Monte-Carlo (Salle Garnier) Opéra de Monte-Carlo
- Opera house: architecture, history, and cultural role Opera house
- Opera seria: the 18th‑century tradition of serious Italian opera Opera seria
- Opera Software
- Opéra-Comique: Paris theatre and company Opéra-Comique
- Operación Puerto: the 2006 Spanish doping investigation Operación Puerto doping case
- Operand (mathematics): definition, types, and usage Operand
- Operant conditioning: principles, history, and practical applications Operant conditioning
- Operating environment (computing) Operating environment
- Operating system (OS): overview, components, history and uses Operating system
- Operation
- Operation (mathematics)
- Operation Barbarossa: the 1941 Axis invasion of the Soviet Union Operation Barbarossa
- Operation Boatswain Memorial — Mount Herzl, Jerusalem Operation Boatswain Memorial
- Operation Car Wash (Operação Lava Jato) — Brazilian corruption investigation Operation Car Wash
- Operation Cornflakes: Allied mail‑based propaganda campaign, February–March 1945 Operation Cornflakes
- Operation Crossbow (1965 film) Operation Crossbow
- Operation Cyclone: U.S. Covert Support to Afghan Mujahideen (1979–1989) Operation Cyclone
- Operation Downfall: Planned Allied invasion of Japan in 1945–46 Operation Downfall
- Operation Eagle Claw (Operation Tabas) — 1980 U.S. hostage rescue attempt Operation Eagle Claw
- Operation Entebbe (Operation Jonathan): 1976 Israeli hostage rescue at Entebbe Airport Operation Entebbe
- Operation Ground and Pound — DragonForce song from Inhuman Rampage Operation Ground and Pound
- Operation Mad Ball (1957 film) Operation Mad Ball
- Operation Meetinghouse — Tokyo firebombing (9–10 March 1945) Operation Meetinghouse
- Operation Mincemeat
- Operation Nickel Grass — 1973 US strategic airlift to Israel Operation Nickel Grass
- Operation Overlord: the 1944 Allied invasion of Western Europe Operation Overlord
- Operation Restore Hope: US-led 1992–93 multinational mission for humanitarian relief in Somalia Operation Restore Hope
- Operation Sea Lion: Nazi Germany’s 1940 plan to invade Britain Operation Sea Lion
- Operation Searchlight: 1971 Pakistani military campaign in East Pakistan Operation Searchlight
- Operation Uphold Democracy (1994–1995) Operation Uphold Democracy
- Operation Yewtree: Metropolitan Police inquiry into historic sexual offences Operation Yewtree
- Neil Armstrong Operations and Checkout Building Operations and Checkout Building
- Operetta: history, characteristics, and legacy of light opera Operetta
- Opeth (Swedish progressive metal band) Opeth
- Opfenbach — municipality in the Lindau district of Bavaria, Germany Opfenbach
- Opfertshofen: village and former municipality in Schaffhausen, Switzerland Opfertshofen
- Opfikon — municipality and city in the canton of Zürich Opfikon
- Opglabbeek (former municipality in Limburg, Belgium) Opglabbeek
- Ophelia (moon)
- Ophidiophobia: fear of snakes — causes, symptoms, and treatment Ophidiophobia
- Ophiolite: exposed sections of oceanic crust and upper mantle Ophiolite
- Ophiotaurus — the Bull-Serpent of Greek Myth Ophiotaurus
- Ophiuchus: the Serpent-Bearer constellation Ophiuchus
- Ophiuchus Supercluster Explosion: the largest known cosmic eruption Ophiuchus Supercluster explosion
- Ophthalmology — the medical and surgical specialty of the eye Ophthalmology
- Ophthalmosauridae: the large‑eyed ichthyosaur family Ophthalmosauridae
- Ophthalmosaurus — a large-eyed Jurassic ichthyosaur Ophthalmosaurus
- Harvestmen (Opiliones): Biology, Diversity, Fossils, and Ecology Opiliones
- Opinion — personal and public views, meaning and uses Opinion
- Opinion poll
- Opio, Alpes-Maritimes — a Provençal commune in southern France Opio, Alpes-Maritimes
- Opioid (drug class): pharmacology, uses, risks, and history Opioid
- Opisthocoelicaudia: a Late Cretaceous Mongolian titanosaur Opisthocoelicaudia
- Opisthokonts: animals, fungi and their unicellular relatives Opisthokont
- Opium: composition, history, uses, and social impacts Opium
- Opium poppy (Papaver somniferum): biology, history, and uses Opium poppy
- Opium Wars: Conflicts that opened China to the West Opium Wars
- Opmeer: overview of the Dutch municipality in North Holland Opmeer
- Opole — regional capital on the Oder River Opole
- Opole Voivodeship
- Opossum (Didelphimorphia): biology, behavior, and ecological role Opossum
- Opp, Alabama
- Oppède: medieval hill village and commune in Vaucluse, Provence-Alpes-Côte d'Azur Oppède
- Oppens (Vaud) — Municipality in Switzerland Oppens
- Oppenweiler
- Oppidum: fortified Iron Age settlements and the birth of pre-Roman towns Oppidum
- Oppligen — municipality in the Bern-Mittelland district, Canton of Bern Oppligen
- Opportunity (MER-B) — NASA's Mars Exploration Rover 'Oppy' Opportunity (rover)
- Opportunity cost: the value of the next-best alternative foregone Opportunity cost
- Opposite
- Opposite number (additive inverse) Opposite number
- Opposition (astronomy and astrology)
- Oppositional Defiant Disorder
- Oppression: causes, forms, history, and responses Oppression
- Oppuurs — village in the province of Antwerp, Belgium Oppuurs
- Oprah Winfrey
- Oprichnina: Ivan IV’s policy of separation and repression (1565–1572) Oprichnina
- Opsin — light-sensitive proteins that mediate photoreception Opsin
- Opsterland
- Opt-out: consumer choices for stopping unwanted marketing and tracking Opt-out
- Optare Alero low‑floor minibus Optare Alero
- Optare CityPacer
- Optare MetroRider
- Optare Solo — low-floor single-deck bus Optare Solo
- Optare Soroco minicoach Optare Soroco
- Optic chiasma: anatomy, development, and clinical significance Optic chiasma
- Optic nerve (cranial nerve II) Optic nerve
- Optic tectum: structure, function, and evolutionary roles Optic tectum
- Optic tract (visual pathway segment) Optic tract
- Optical character recognition (OCR): technology, methods and applications Optical character recognition
- Optical depth (attenuation of radiation) Optical depth
- Optical disc drive
- Optical fiber thin fiber of glass or plastic that can carry light
- Optical filter: types, performance and common applications Optical filter
- Optical gain medium
- Optical illusion: how the brain creates misleading visual perceptions Optical illusion
- Optical printer — film re‑photography device for effects, copying and restoration Optical printer
- Optics: The Science and Technology of Light Optics
- Optima — humanist sans‑serif typeface by Hermann Zapf Optima
- Optimism: meaning, forms, history, psychology, and practical effects Optimism
- Optimization (computer science)
- Optimus Prime — leader of the Autobots Optimus Prime
- Opus Dei (Prelature of the Holy Cross and Opus Dei) Opus Dei
- Opus number: definition, history, and how musical works are identified Opus number
- Opwijk — Municipality in Flemish Brabant, Belgium Opwijk
- Oquawka, Illinois
- OR gate (inclusive disjunction) in boolean logic and digital circuits OR gate
- Ora — debut studio album by Rita Ora (2012) Ora (Rita Ora album)
- Ora Namir — Israeli politician, government minister and diplomat Ora Namir
- Oraàs — a commune in Pyrénées-Atlantiques, France Oraàs
- Oracle (Ancient Greek prophetic institution) Oracle
- Oracle Corporation
- Oracle Database
- Oracle of Delphi: The Pythia and the Temple of Apollo Oracle of Delphi
- Oracle Park
- Oradea — city in Bihor County, Romania Oradea
- Orai, Uttar Pradesh Orai
- Orain — commune in Côte-d'Or, France Orain
- Orainville — commune in Aisne, northern France Orainville
- Orakzai District
- Oral cancer: causes, signs, diagnosis and treatment Oral cancer
- Oral history: recording and preserving spoken memories Oral history
- Oral law: traditions, transmission, and role in societies Oral law
- Oral rehydration therapy: purpose, composition, history, and use Oral rehydration therapy
- Oral sex: definition, types, health considerations, and social context Oral sex
- Oral tradition: transmission of stories, law, and knowledge by voice Oral tradition