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Ob (鄂畢河, rivers_lake_centerlines:24)
The Ob (Обь; 鄂畢河) is a major west Siberian river formed by the Biya and Katun near Biysk, flowing north to the Gulf of Ob; third largest Arctic river by discharge, chief tributary the Irtysh.
Ocean
The ocean is the interconnected salt-water body covering about 70% of Earth's surface, holding 97% of its water, absorbing roughly 30% of CO2 emissions and 93% of excess heat, and governed largely under UNCLOS.
Ocean Beach, San Francisco
Ocean Beach is San Francisco's roughly five-mile Pacific shoreline south of the Cliff House, backed by the Great Highway and O'Shaughnessy seawall, an erosion hot spot studied by USGS, and fronted since 2025 by the car-free Sunset Dunes park.
Ocean temperature
Ocean temperature: seawater temperature from surface to depth, a key climate indicator. The ocean stores about 90% of excess heat; 2025 upper-2000 m ocean heat content set a record, up ~23 zettajoules over 2024.
Oceanic basin
Oceanic basin: the large crustal depressions holding the Pacific, Atlantic, Indian, Southern, and Arctic oceans; defined by ridges, trenches, abyssal plains, and only about 27-29% mapped.
Oceanic trench
Oceanic trench: long, narrow seafloor depressions at subduction zones, the deepest parts of the ocean. 38 known, 28 in the Pacific; deepest is Challenger Deep, Mariana Trench, about 10,924 m.
Ocmulgee River
The Ocmulgee River, whose name comes from a Hitchiti word meaning "where water boils up," is a 241-mile river in central Georgia and the westernmost major tributary of the Altamaha system.
Oder
Oder (Odra, Latin Viadus/Viadua): Central European river, 840-854 km, rising in the Czech Republic, flowing through Poland, forming the German-Polish border since 1945, and reaching the Baltic via the Szczecin Lagoon; site of the 1997 flood and the 2022 Prymnesium parvum fish-kill disaster.
Ogooué
The Ogooué (Ogowe) is Gabon's principal river, rising in the Republic of the Congo and flowing about 920–1,200 km to the Atlantic. Second-largest Central African river by discharge (~4,750 m³/s), bimodal floods, largely undammed, 308 fish species.
Ogooue - Nyanga - Kouilou - Niari
Ogooue-Nyanga-Kouilou-Niari, FEOW freshwater ecoregion 532, covers the Ogooué, Kouilou-Niari, and Nyanga river basins in Gabon, Congo, DRC, and Cabinda; about a quarter of its fishes are endemic.
Ogopogo
Ogopogo, from Syilx n'x̌ax̌aitkʷ (spirit of the lake), is the legendary lake monster of Okanagan Lake, British Columbia; named after a 1924 music-hall song, with no confirmed physical evidence, and copyright transferred to the Okanagan Nation Alliance in 2021.
Ohio River
The Ohio River, called O-Y-O ("the great river") by the Iroquois, is a 981-mile tributary of the Mississippi formed at Pittsburgh by the Allegheny and Monongahela Rivers, ending at Cairo, Illinois, with 20 navigation dams and over 230 million tons of annual cargo.
Oka (اوكا)
Oka (Ока), river in European Russia, largest right tributary of the Volga; 1,500 km, 245,000 km² basin, rises in Oryol Oblast, joins the Volga at Nizhny Novgorod; heavily polluted in middle reaches.
Okavango (river, Botswana)
The Okavango River, called the Cubango in Angola, is a southwestern African river about 1,600 km long that rises on the Bie Plateau and ends in Botswana's inland Okavango Delta.
Okavango Delta
Okavango Delta System: endorheic inland delta (alluvial fan) in north-western Botswana, fed by Angola's Okavango River; Ramsar site 1996, UNESCO World Heritage Site 2014; Africa's largest endorheic delta.
Okefenokee Swamp
Okefenokee Swamp: a nearly 700-square-mile blackwater peatland in southeast Georgia, headwaters of the Suwannee and St. Marys rivers, a National Wildlife Refuge since 1937 and a UNESCO World Heritage site since 2026.
Okhotsk Coast
Okhotsk Coast (FEOW ecoregion 614): Russian Far East freshwater ecoregion of short coastal rivers draining the Sea of Okhotsk coast from Gizhiginskaya Guba Bay to the Kiran River, marked by charr diversity and absence of Coregonidae and Amur fauna.
Old Croghan Man
Old Croghan Man: Irish Early Iron Age bog body (c. 362–175 BC) found June 2003 in Clonearl Bog, Co. Offaly; ~198 cm tall, manicured nails, ritual killing, displayed at the National Museum of Ireland, Dublin.
Old Faithful
Old Faithful, a cone geyser in Yellowstone National Park's Upper Geyser Basin, Wyoming, erupts boiling water up to 184 feet every 54 minutes to about two hours.
Old Wives
Old Wives Lake (Cree Notekeu, "old woman") is a large shallow saline lake in southern Saskatchewan near Moose Jaw, in a closed internal-drainage basin, a migratory bird sanctuary and hemispheric shorebird reserve formerly named Johnstone Lake.
Olenyok
Olenyok (Оленёк), a 2,270 km river in Yakutia and Krasnoyarsk Krai, Russia, flowing from the Vilyuy Plateau to Olenyok Bay of the Laptev Sea; discharge 1,210 m³/s, basin 219,000 km², site of the 1633 Olenyok winter quarters.
Olenyok Gulf
Olenyok Gulf (Оленёкский залив), a shallow gulf of the Laptev Sea in Yakutia, Russia, west of the Lena Delta; 65 km long, about 130 km wide, up to 15 m deep, ice-covered from mid-October to June.
Olvassuo Mires
Olvassuo Mires (Finnish: Olvassuo) is a large, mostly pristine aapa mire complex in northern Finland, designated a Ramsar site in 2004 and known as a breeding ground for mire birds.
Olyokma
Rising on the northern slope of the Yablonovy Range, the Olyokma (Олёкма, Olekma) is a right tributary of the Lena and flows through Amur Oblast and Yakutia.
Omolon
Omolon (Омолон), river in Magadan Oblast, Chukotka, and Yakutia, Russia; longest right tributary of the Kolyma, 1,114 km, basin ~113,000 km², navigable ~595 km, village of Omolon on its banks.
Onega
Onega (Onego, Онежское озеро), Europe's second largest lake, in Karelia, Russia; ~9,720–9,890 km², Svir River outlet to Ladoga, glacial origin, Onega petroglyphs.
Onon
Onon (Онон гол, 鄂嫩河, historic 斡難河) is a 1,032 km river rising in Mongolia's Khentii mountains, joining the Ingoda to form the Shilka of the Amur basin; birthplace region of Genghis Khan.
Onondaga Lake
Onondaga Lake, near Syracuse, New York, is a 4.6-square-mile lake, once among America's most polluted, a Superfund site since 1994, sacred to the Onondaga Nation, now recovering after Honeywell's 2014 dredging.
Onsen (温泉)
Onsen (温泉) are Japanese hot springs and the bathing facilities built on them, legally defined by the 1948 Hot Spring Law, distinct from sento public baths, with over 2,300 facilities, nude bathing etiquette, and a toji spa-cure tradition.
Ontong Java Plateau
Ontong Java Plateau, largest oceanic plateau on Earth, western Pacific near Solomon Islands; formed ~120 Ma, area over 1.5 million km2, 30-35 km crust, submarine tholeiitic basalts, origin debated between thermochemical plume, spreading, and impact.