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- Cathy O'Dowd (born 1968) South Africa. First female ascent of Everest from both north and south (1999). Fourth female ascent Lhotse (2000).
- Oh Eun-Sun (born 1966) South Korea. First Korean woman to climb Seven Summits. Controversy over eight-thousanders claim.
- Juanito Oiarzabal (born 1956) Basque (Spain). All eight-thousanders without supplementary oxygen. Record 24 ascents of eight-thousanders.
- Adam Ondra (born 1993) Czech Republic. Youngest climber to climb 5.14d.
- Dan Osman (1963–1998) USA. Rock climber, soloist. Killed whilst attempting his new sport of rope jumping.
- James Outram (1864–1925) Canada. First ascent of Mount Assiniboine.
Read more about this topic: List Of Climbers And Mountaineers