O1 or O-1 may refer to:
Aircraft Cessna O-1 Bird Dog, an observation aircraft manufactured by Cessna O-1 Curtiss Falcon, an observation aircraft manufactured by the Curtiss Aircraft Company O-1 Airship, an Italian manufactured semi-rigid airship operated by the US Navy
Music O1 (album), by Hiroyuki Sawano, 2015 O1, a 2000 album by Son of Dave
Military O-1 or fenrik, a second lieutenant rank in Norway The pay grade for ensign and second lieutenant ranks in the U.S. uniformed services USS O-1 (SS-62), lead ship of the United States O class submarines HNLMS O 1, a 1905 Royal Netherlands Navy O class submarine
Science and technology , or atomic oxygen O1 type star, a subclass of O-type star O-1 tool steel, a type of oil-hardening tool steel Haplogroup O1 (Y-DNA), a Y-DNA haplogroup O1: an EEG electrode site according to the 10–20 system O(1), time complexity expressed using Big O notation OpenAI o1, AI model released by OpenAI
Other O-1 visa, a variety of the U.S. O visa, allowing for temporary immigration for work purposes for individuals of extraordinary ability or achievement in certain areas Otoyol 1, a motorway in Turkey Ö1, an Austrian radio station O1 Communications, Inc., a California telecommunications company Orchestra ONE (O1), an experimental youth orchestra based in Maidstone, United Kingdom GNR Class O1, a class of British 2-8-0 steam locomotive designed by Nigel Gresley (later redesignated Class O3) LNER Thompson Class O1, a class of British 2-8-0 steam locomotive designed by Edward Thompson and rebuilt from Class O4
See also 01 (disambiguation) 1O (disambiguation)