O1

O1 or O-1 may refer to:

Aircraft

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

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

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