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- ObergruppenführerParamilitary rank in Nazi Germany
- Oberkommando der WehrmachtSupreme military command and control office of Nazi Germany during World War II
- Occupation of German SamoaWorld War I occupation by New Zealand
- Occupation of Poland (1939–1945)During World War II, Poland was occupied by Nazi Germany and the Soviet Union (and Slovaki
- Occupation of the Baltic statesSoviet and Nazi German occupation (1940–1991)
- Occupation of the Rhineland1918–1930 occupation by the WWI Allies
- Occupation of the Ruhr1923–1925 occupation of Germany's Ruhr district
- October RevolutionSecond of two 1917 revolutions in Russia
- Oder–Neisse lineGerman–Polish border since the end of World War II
- Office of Fair Housing and Equal OpportunityThe Office of Fair Housing and Equal Opportunity (FHEO) is an agency within the United Sta
- Official names of the Soviet UnionThe official names of the Soviet Union, officially known as the Union of Soviet Socialist
- Ōgaki DomainŌgaki Domain (大垣藩, Ōgaki-han) was a fudai feudal domain under the Tokugawa shogunate of Ed
- Oklahoma Enabling ActThe Enabling Act of 1906, in its first part, empowered the people residing in Indian Terri
- Oklahoma National GuardThe Oklahoma National Guard, a division of the Oklahoma Military Department, is the compon
- Oklahoma Tribal Statistical AreaOklahoma Tribal Statistical Area is a statistical entity identified and delineated by fede
- Old Guangxi cliqueThe Old Guangxi Clique (simplified Chinese: 旧桂系; traditional Chinese: 舊桂系; pinyin: Jiù Guì
- Old State Capitol State Historic SiteThe Old State Capitol State Historic Site, in Springfield, Illinois, is a former capitol b
- Omaha BeachWWII amphibious landing zone in France
- Omaha Beach (horse)Omaha Beach (foaled April 24, 2016) is an American Thoroughbred racehorse who was the morn
- Omani EmpireOmani maritime empire (1696–1856)
- On the Cult of Personality and Its Consequences1956 speech by Soviet leader Nikita Khrushchev
- On the Way of ResurrectionOn the Way of Resurrection (Arabic: فِي سَبِيلِ البعث, Fī-Sabīl al-Baʿth) is a political l
- One China, Many PathsOne China, Many Paths, edited by Chaohua Wang. A collection of essays by Chinese thinkers,
- Operation Achse1943 German campaign to disarm Italy following its armistice to the Allies during WWII
- Operation AgasOperation Agas was a series of reconnaissance operations carried out by Australia's Z Spec
- Operation AnimalsOperation Animals was a World War II mission by the British Special Operations Executive (
- Operation Backfire (World War II)Operation Backfire was a military scientific operation of the Western Allies during and af
- Operation Bagration1944 Soviet military offensive during WW2
- Operation BalsamOperation Balsam was a British naval operation in World War II, from 10–20 June 1945, unde
- Operation BambergOperation Bamberg was a Nazi security warfare operation during the Occupation of Belarus b
- Operation Banquet (Padang)Operation Banquet was a British naval operation in the Second World War, commanded by Rear
- Operation Barbarossa1941 Axis invasion of the Soviet Union during WWII
- Operation BertramOperation Bertram was a Second World War deception operation practised by the Allied force
- Operation Big SwitchOperation Big Switch was the repatriation of all remaining prisoners of the Korean War. Ce
- Operation BullseyeOperation Bullseye was the code-name of the first Special Operations Executive (SOE) missi
- Operation ChastityOperation Chastity was a World War II plan by the Allies to seize Quiberon Bay, France, en
- Operation Collar (convoy)Operation Collar (12–29 November 1940) was a small, fast three-ship convoy during the Seco
- Operation CollieOperation Collie was a British naval operation in World War II, from 5–11 July 1945. Its o
- Operation CondorUS-backed repression campaign in South America
- Operation Crossroads1946 nuclear weapon tests at Bikini Atoll
- Operation DonnerschlagDuring World War II, the German military planned or undertook an operation named Donnersch
- Operation Eager GlacierOperation Eager Glacier was a secret U.S. effort to spy on Iran with aircraft in 1987 and
- Operation FaustschlagLast action on the Eastern Front, WW1
- Operation Flagpole (World War II)Operation Flagpole was part of the run-up to Operation Torch, the planned Allied invasion
- Operation FrithamOperation Fritham (30 April – 14 May 1942) was an Allied military operation during the Sec
- Operation Jungle1945–1955 British MI6 program to infiltrate its agents into Poland and Baltic states
- Operation LeaderOperation Leader was an air attack conducted against German shipping in the vicinity of Bo
- Operation Lentil (Caucasus)1944 Soviet ethnic cleansing and genocide
- Operation LightOperation Light was a British naval operation in World War II under the command of Rear Ad
- Operation LiveryOperation Livery was a series of naval air strikes on northern Japanese occupied Malaya an
- Operation LUSTYOperation LUSTY ("Luftwaffe Secret Technology") was the United States Army Air Forces' eff
- Operation Market Garden1944 World War II military operation
- Operation MatterhornWar campaign in WWII
- Operation Michael1918 German offensive during World War I
- Operation OsoaviakhimPost WWII Soviet secret operation
- Operation Panzerfaust1944 Nazi German military operation to replace Hungary's leadership with loyalists
- Operation PaperclipSecret post-WWII United States program
- Operation PokpungKorean War offensive
- Operation Provide ComfortOperation Provide Comfort and Provide Comfort II were military operations initiated by the
- Operation Rat KillerOperation Rat Killer was a Korean War operation carried out by Republic of Korea forces an
- Operation Ratweek (1944)Operation Ratweek was a series of coordinated attacks on the Axis forces' communication li
- Operation ReinhardCode name for the creation of German extermination camps in Poland in World War II
- Operation ReservistOperation Reservist was an Allied military operation during the Second World War. Part of
- Operation RobsonOperation Robson (20 December 1944) was the first of a series of aerial operations, Operat
- Operation RolandOperation Roland was a local German offensive inside the Soviet Union during the Second Wo
- Operation SaxifrageDuring World War II, Operation Saxifrage was a raid by four small Special Air Service team
- Operation SemutOperation Semut was a series of reconnaissance operations carried out by Australia's Z Spe
- Operation SlapstickOperation Slapstick was the code name for a British landing from the sea at the Italian po
- Operation StabOperation Stab was a British naval deception during the Second World War to distract Japan
- Operation TarbrushOperation Tarbrush was the name of a series of British Commando raids during the Second Wo
- Operation TorchAllied landing operations in French North Africa during World War II
- Operation UndergoAllied siege and capture of Calais in 1944
- Operation ValuableWestern covert paramilitary operation
- Operation VerminOperation Vermin (Aktion Ungeziefer) and Operation Consolidation (Betriebskonsolidierung,
- Operation Weserübung1940 German invasion of Denmark and Norway in WWII
- Opium Wars19th-century conflicts between China and European powers
- Opposing forces in the Polish September CampaignThe main opposing forces in the Polish September Campaign, which marked the beginning of t
- Opposition to the overthrow of the Hawaiian KingdomOpposition to the overthrow of the Hawaiian Kingdom took several forms. Following the over
- Orange Free State1854–1902 Boer republic in Southern Africa
- Order No. 2271942 military command issued by Soviet Union leader Joseph Stalin
- Order Police battalionsMilitarised police units of Nazi Germany
- Oregon boundary disputeEarly 19th century US–UK boundary dispute
- Oregon TrailHistoric migration route spanning Independence, MO–Oregon City, OR
- Organic Articles1802 French law about religion
- Organisation of the League of NationsIntergovernmental organization
- Organization of Yugoslav NationalistsPolitical organisation in Yugoslavia, 1921–1929
- Organizations of the Iranian RevolutionMany organizations, parties and guerrilla groups were involved in the Iranian Revolution.
- Organized crime in TaiwanOrganized crime in Taiwan refers to the activities of criminal syndicates in Taiwan. The o
- OrgburoOrganisational office of the Communist Party of the Soviet Union
- Orzeł incidentThe Orzeł incident occurred at the beginning of World War II in September 1939, when the i
- Oskar von HutierOskar Emil von Hutier (27 August 1857 – 5 December 1934) was a German general during the F
- OstpolitikNormalization of relations between West Germany and Eastern Europe
- Osvald GroupThe Osvald Group was a Norwegian organisation that was the most active World War II resist
- Other investigations of the Dreyfus affairAfter Major Georges Picquart's exile to Tunisia others took up the cause of the Alfred Dre
- Otto Philipp BraunOtto Philipp Braun (13 December 1798 – 24 July 1869, also known as Felipe Braun during his
- Otto RugeOtto Ruge (9 January 1882 – 15 August 1961) was a Norwegian general. Ruge was Commander-in
- Otto von MoserOtto von Moser (31 March 1860 – 11 October 1931) was a German army officer, originally fro
- Otto von StülpnagelOtto Edwin von Stülpnagel (16 June 1878 – 6 February 1948) was a German military commander
- Ottoman Army (1861–1922)1861–1922 land warfare branch of the Ottoman Empire's military
- Ottoman entry into World War IEntrance of the Ottoman Empire into the First World War
- Ottoman InterregnumCivil war in the early 15th century Ottoman Empire
- OttomanismConcept that emerged towards the end of the Tanzimat period
- Ouémé DepartmentOuémé is one of the twelve departments of Benin, containing the capital of the country Por
- Ovambo UprisingOvambo rebellion against Portuguese colonial rule
- Overthrow of the Hawaiian Kingdom1893 government overthrow
- Owen Tudor BoydAir Marshal Owen Tudor Boyd, CB, OBE, MC, AFC (30 August 1889 – 5 August 1944) was a Briti