3,546 articles
Orkney
Orkney, a Scottish archipelago of 68+ islands (990 km², 16 inhabited) north of mainland Scotland, with capital Kirkwall, Norse heritage, the Heart of Neolithic Orkney World Heritage Site, and the EMEC marine energy test centre.
Ornatenton Formation
Ornatenton Formation: Middle Jurassic (late Bathonian to Early Oxfordian) shallow-marine mudstone and siltstone unit of the North German Dogger Group and South German Jura, source of Wiehenvenator.
Oros Ossa (Kissavos)
Oros Ossa (Kissavos, Όσσα/Κίσσαβος) is a 1,978 m mountain in eastern Thessaly, Greece, between Olympus and Pelion, divided from Olympus by the Tempe gorge; Natura 2000 Aesthetic Forest, giants' mythology, blueschist geology.
Ortler / Ortlés
Ortler (Italian: Ortles), 3,905 m, is the highest mountain of South Tyrol and of the Eastern Alps outside the Bernina Range. First climbed 27 September 1804 by Josef Pichler; its north face is the Eastern Alps' largest ice face.
Orust
Orust is an island and municipality in Bohuslän, Västra Götaland, Sweden; the largest island on the west coast (386 km²), seat Henån, 15,363 inhabitants in 2025, known for boatbuilding and fishing villages.
Osa Peninsula
Osa Peninsula, southwestern Costa Rica Pacific peninsula (~1,500 km2) harboring 2.5% of global terrestrial biodiversity, Corcovado National Park, the Osa Mélange, and Cocos Ridge subduction uplift.
Oscar II Coast
Oscar II Coast, also Kong Oscar II Land, is the east coast of Graham Land, Antarctic Peninsula, between Cape Fairweather and Cape Alexander, charted by C. A. Larsen in 1893 and named for King Oscar II of Sweden and Norway.
Oscar Peak
Oscar Peak (Pic Oscar) is a 2,336 m summit in the Nass Group of the Coast Mountains, British Columbia, named for settler Oscar Olander, first climbed in 1925 by Broden and Olander, with 2,099 m prominence.
Oshtoran Kuh
Oshtoran Kuh (اشترانکوه), the 'mountain of camels', is a 4,081 m ultra-prominent peak in Lorestan province's central Zagros, Iran's highest in Lorestan, with glacial valleys, Gahar lakes, and a 1970 protected area.
Osinovo Formation
The Osinovo Formation is a Paleogene quartz-sand and sandstone unit in the Cis-Donets zone of the southern Russian Plate and Luhansk region, eastern Ukraine, best known from the Ikovo stratotype.
Osterrieth Range
The Osterrieth Range is a mountain range extending northeast to southwest along the southeast coast of Anvers Island, in the Palmer Archipelago off the Antarctic Peninsula.
Ostrov Bel’kovskiy
Ostrov Bel'kovskiy (Belkovsky Island, Остров Бельковский) is the westernmost of the Anzhu Islands in the New Siberian Islands, Russia, west of Kotelny across the Strait of Zarya; uninhabited tundra up to 127 m with walrus haulouts and Permian-Triassic trap magmatism.
Ostrov Bol’shevik
Ostrov Bol'shevik (остров Большевик), Bolshevik Island, is the southernmost, second-largest island of Severnaya Zemlya, Russia: about 11,300 km², 935 m high, ~30% glaciated, separated from Taymyr by Vilkitsky Strait, with accelerating glacier retreat and Cape Baranov research station.
Ostrov Bol’shoy Shantar
Bolshoy Shantar (Большой Шантар) is the largest island of the Shantar archipelago in the western Sea of Okhotsk, Khabarovsk Krai, Russia; about 1,790 km², with brackish Lake Bolshoe, Yakshina Bay meteorological station, and inclusion in Shantar Islands National Park since 2013.
Ostrov Gallya
Ostrov Gallya (Остров Галля, Hall Island) is the fifth largest island of Franz Josef Land, Russia, about 1049-1200 km², 93.8% glacier-covered, highest point 502 m, discovered 1874 by the Austro-Hungarian Tegetthoff expedition.
Ostrov Gukera
Ostrov Gukera (остров Гукера, Hooker Island) is an island in the Franz Josef Land archipelago, best known for Bukhta Tikhaya, the first Soviet polar station on the archipelago, opened in 1929.
Ostrov Kamenka
Ostrov Kamenka (Остров Каменка), also rendered Ostrov Kamenka-Sar, is an inhabited island in western Lake Pskov in Pechorsky District, Pskov Oblast, northwestern Russia, part of a protected natural monument designated in 2007.
Ostrov Matar
Ostrov Matar (остров Матар) is a small uninhabited island in the Anzhu Islands of the New Siberian archipelago, East Siberian Sea, Yakutia, Russia; about 8 km², 5 km long, near Bunge Land and Zheleznyakov Island.
Ostrov Mezhdusharskiy
Ostrov Mezhdusharskiy (Остров Междушарский), also Mezhdusharski, is the third largest island of Russia's Novaya Zemlya archipelago, 742 km², in the Barents Sea west of Yuzhny Island, separated by Kostin Shar strait.
Ostrov Pioner
Ostrov Pioner (Остров Пионер, Pioneer Island) is an uninhabited 1,527 km² island in northwestern Severnaya Zemlya, Kara Sea, Krasnoyarsk Krai, Russia; highest point 385 m, discovered by Ushakov and Urvantsev in 1930–32.
Ostrov Severnyy
Ostrov Severnyy (Остров Северный), or Severny Island, is the northern of the two large islands of Novaya Zemlya in the Russian Arctic, belonging to Arkhangelsk Oblast.
Ostrov Sibiryakova
Ostrov Sibiryakova (Остров Сибирякова), or Kuz'kin Island, is an ~800 km² flat tundra island in the southern Kara Sea at the entrance to the Yenisei Gulf, Krasnoyarsk Krai, named by Nordenskiöld in 1878 and part of the Great Arctic Reserve.
Ostrov Solsberi
Ostrov Solsberi (Остров Солсбери), also called Salisbury Island, is a largely glaciated island in central Franz Josef Land, Russia, the largest of the Zichy Land group.
Ostrov Vaygach
Ostrov Vaygach (Остров Вайгач, Vaygach Island), Nenets 'Holy Land' (Khebidya-Ya), an Arctic island between the Barents and Kara seas in Nenets Autonomous Okrug; 3,383-3,400 km2, sacred Nenets sites, 1930s OGPU lead-zinc mining, zakaznik since 2007.
Ostrov Zemlya Aleksandry
Ostrov Zemlya Aleksandry (Alexandra Land, Земля Александры) is the westernmost large island of Franz Josef Land, Russia, in the Barents Sea; about 1,000 km², mostly ice-covered, site of Nagurskoye airbase and the Arctic Trefoil military base.
Ostry
Ostry (Ostraya, Ostriy), GVP 300680, is a dormant 2,552 m stratovolcano in the northern Sredinny Range, central Kamchatka, Russia; last eruption about 2050 BCE from flank Cone X; distinct from Ostry Tolbachik.
Ōsumi Islands
Ōsumi Islands (大隅諸島), also called Osumi-shoto, are Kagoshima Prefecture islands south of Kyushu: Yakushima (World Heritage cedar forests), Tanegashima (space center, surfing), and the three small Mishima-mura islands.
Otgon Tenger Uul
Otgon Tenger Uul (Otgontenger, Отгонтэнгэр, "Youngest Sky") is the highest peak of Mongolia's Khangai Mountains, about 4,008–4,010 m, an ultra-prominent sacred mountain with the range's only glacier.
Ouachita Highlands
The Ouachita Highlands are the Ouachita Mountains of western Arkansas and southeastern Oklahoma, which with the Ozark Plateau form the U.S. Interior Highlands, with east-west ridges and elevations up to 839 m.
Ouachita Mountains
Ouachita Mountains: east-west fold-and-thrust belt in west-central Arkansas and southeastern Oklahoma, formed by the late Paleozoic Ouachita Orogeny (soft collision, ~280 Ma), part of the Appalachian-Ouachita-Marathon chain, home to Hot Springs National Park and Ouachita National Forest.