(plural first officers)
- The copilot of an aircraft.
- The chief mate of a merchant ship.
- The executive officer, second in command of a naval vessel.
- The most senior civil servant in the United States Foreign Service, with responsibility for running an embassy under the authority of and reporting to the ambassador.
- Air Transport Auxiliary rank corresponding to Flight Lieutenant in the Royal Air Force.
- Women's Royal Naval Service rank corresponding to Lieutenant-Commander in the Royal Navy.