1,303 articles
Nuremberg trials
The Nuremberg trials were post-WWII tribunals in Nuremberg, Germany, where the International Military Tribunal tried 22 Nazi leaders for crimes against peace, war crimes, and crimes against humanity, convicting 19 and sentencing 12 to death, followed by twelve Subsequent Nuremberg Trials.
Obligations of mandatory powers under the League mandates system
The obligations of mandatory powers were the legal duties under Article 22 of the League Covenant requiring the well-being of mandated territories' inhabitants, bans on slavery, and annual reports.
Occupied Enemy Territory Administration
Occupied Enemy Territory Administration (OETA): British military government of former Ottoman Palestine, Lebanon, Syria, and Transjordan from late 1917, divided into South, North, and East zones, replaced by League mandates in 1920.
Oder–Neisse line
Oder–Neisse line: the German-Polish border along the Oder and Neisse rivers, set provisionally at Potsdam in 1945, recognized by the GDR in 1950, by West Germany in the 1970 Treaty of Warsaw, and made final in 1990.
Odious debt
Odious debt, also called illegitimate debt, is a doctrine systematized by Alexander Sack in 1927 holding that debts incurred by despotic regimes without popular consent, benefit, or creditor innocence are unenforceable, though no tribunal has applied it.
Office of the Ombudsperson for the ISIL and Al-Qaida List
UN office reviewing delisting petitions for the ISIL (Da'esh) and Al-Qaida Sanctions List, created by resolution 1904 (2009); issues comprehensive reports and recommendations, mandate extended to 17 June 2027.
Office of the United Nations High Commissioner for Human Rights
OHCHR (Office of the United Nations High Commissioner for Human Rights) is the UN Secretariat department promoting human rights, established December 1993; serves the Human Rights Council, UPR, treaty bodies, and special procedures, led by Volker Türk from Geneva.
Oil Platforms case
ICJ case in which Iran sued the United States over the 1987 and 1988 destruction of three National Iranian Oil Company offshore complexes; the Court in 2003 rejected both claims for reparation, finding no armed attack justified the strikes and no treaty commerce was impeded.
Oil-for-Food Programme
The Oil-for-Food Programme (OFFP) was a UN program under Security Council resolution 986 (1995) letting sanctioned Iraq sell oil for food, medicine, and civilian goods; it ran December 1996 to November 2003, moved over $67 billion in oil revenue, and was later investigated for corruption by the Volcker Committee.
Omurtag of Bulgaria
Khan Omurtag (Омуртаг), also spelled Omortag, was khan of Bulgaria from 814 or 815 to 831, known as "the Builder" for rebuilding his capital Pliska in stone.
Ongoing and stalled modern treaty negotiations in Canada
Ongoing and stalled modern treaty negotiations in Canada are unfinished land claim and self-government talks between Indigenous nations, the federal government, and provinces, with about 100 tables open.
Open Door Policy
The Open Door Policy was U.S. Secretary of State John Hay's 1899-1900 notes to the great powers calling for equal trading opportunity in China and respect for China's territorial and administrative integrity, the basis of U.S. Far East policy into the mid-20th century.
Operation Deny Flight
Operation Deny Flight was a NATO air operation from 1993 to 1995 that enforced a UN no-fly zone over Bosnia and provided close air support to UN peacekeepers.
Operation Restore Hope
Operation Restore Hope was the United States-led humanitarian intervention in Somalia from December 1992 to May 1993, deploying about 37,000 troops to secure famine relief.
Opinio juris sive necessitatis
Opinio juris sive necessitatis (opinio juris), Latin for 'an opinion of law or necessity', is the subjective element of customary international law: states must act from a sense of legal right or obligation, not mere usage.
Opposition to the overthrow of the Hawaiian Kingdom
Opposition to the overthrow of the Hawaiian Kingdom: Native Hawaiian resistance after the January 17, 1893 coup against Queen Liliʻuokalani, including the 1895 Wilcox Rebellion, the 21,269-signature Kūʻē Petitions of Hui Aloha ʻĀina, and Senate lobbying that defeated the 1897 annexation treaty.
Opt-outs in the European Union
Opt-outs in the European Union are treaty-based exemptions letting member states stay outside specific EU policies, notably Denmark's four Maastricht opt-outs (euro, defense, citizenship, justice) and the UK's EMU, Social Protocol, and Schengen exemptions.
Optional Protocol on the Involvement of Children in Armed Conflict
The Optional Protocol on the Involvement of Children in Armed Conflict (OPAC), a 2000 treaty, prohibits compulsory recruitment of persons under 18 in armed forces.
Optional Protocol to the Convention on Special Missions
The Optional Protocol to the Convention on Special Missions is a 1969 treaty giving the International Court of Justice compulsory jurisdiction over disputes between its parties.
Optional Protocol to the Vienna Convention on Diplomatic Relations
Two treaties adopted at Vienna on 18 April 1961 alongside the Vienna Convention on Diplomatic Relations, covering compulsory ICJ dispute settlement and diplomats' nationality; both entered into force on 24 April 1964.
Oregon Treaty
Oregon Treaty (Treaty of Oregon, 1846): Anglo-American treaty signed June 15, 1846, setting the 49th parallel as the Pacific Northwest border, ending joint occupation, and leaving the San Juan Islands dispute for 1872 arbitration.
Otfried Nippold
Otfried Nippold (1864–1938) was a German-Swiss jurist and peace campaigner, a professor of international law at Bern who presided over the Saar's League-administered supreme courts from 1920 to 1934.
Ottawa Treaty
The Ottawa Treaty (Anti-Personnel Mine Ban Convention, APMBC) is a 1997 treaty banning anti-personnel landmines, in force 1 March 1999, with 162 parties; major non-parties include the US, Russia, and China, and several European states withdrew in 2025–2026.
Ottoman–Venetian peace treaty (1419)
The Ottoman–Venetian peace treaty of 1419, concluded at Adrianople on 6 November 1419 between Sultan Mehmed I and Venice, ended their war, confirmed Venetian holdings in the Aegean and Balkans, and governed trade by sea.
Outer Space Treaty
The Outer Space Treaty (1967), formally the Treaty on Principles Governing Activities of States in the Exploration and Use of Outer Space, is the foundation of international space law: no WMD in orbit, no national appropriation, states responsible for private space activities.
P5+1
P5+1: China, France, Germany, Russia, UK, and US (UNSC P5 plus Germany), the six-power grouping that negotiated Iran's nuclear program, produced the 2015 JCPOA, and later operated as P4+1/E3/EU+2 after the 2018 US withdrawal.
Pact of Steel
The Pact of Steel (Stahlpakt, Patto d'Acciaio), formally the Pact of Friendship and Alliance, was the German-Italian military alliance signed in Berlin on May 22, 1939, distinct from the 1936 Anti-Comintern Pact and the 1940 Tripartite Pact.
Pacta sunt servanda
Pacta sunt servanda, Latin for "agreements must be kept," is the principle that every treaty in force binds its parties and must be performed in good faith.
Pacta tertiis nec nocent nec prosunt
Pacta tertiis nec nocent nec prosunt is the treaty-law rule that treaties bind only consenting States, codified in Article 34 of the 1969 Vienna Convention.
Palestinian war crimes
Palestinian war crimes: alleged laws-of-war violations by Hamas and other Palestinian armed groups, notably the 7 October 2023 attack, hostage-taking, and killings, as documented by UN inquiries and charged by the ICC, alongside parallel findings against Israeli forces.