O1

O1 or O-1 may refer to:

Aircraft

O-1 Bird Dog, an observation aircraft manufactured by Cessna
O-1 Falcon, an observation aircraft manufactured by the Curtiss Aircraft Company
O-1 Airship, an Italian manufactured semi-rigid airship operated by the US Navy

NATO Rank O-1
O-1, the pay grade for the following officer ranks in the U.S. uniformed services:
- Second Lieutenant in the Army, Air Force, and Marine Corps
- Ensign in the Navy, Coast Guard, Public Health Service Commissioned Corps, and NOAA Commissioned Officer Corps
USS O-1 (SS-62), lead ship of the United States O class submarines
HNLMS O 1, a Royal Netherlands Navy O class submarine

O1 type star, a subclass of O-type star
O-1 tool steel, a type of oil-hardening tool steel
O-1 visa
Haplogroup O1 (Y-DNA), a Y-DNA haplogroup
SP&S Class O-1, a steam locomotives class
Otoyol 1, a Turkish highway around Istanbul
Ö1, an Austrian radio station
O1 Communications, Inc., a California telecommunications company
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