FSO may refer to:
- Fabryka Samochodów Osobowych, a Polish automotive company
- Facility Security Officer, an industrial security job
- Familia spiritualis opus, FSO, a Roman Catholic "spiritual community", a new type of community with less strict rules than the traditional orders. Members add "FSO" behind their names, example "Father Peter Schmidt, FSO".
- Federal Protective Service (Russia), responsible for the security of Russian state officials
- FileSystemObject (FSO), an object model in Windows Script Host. See also Scripting.FileSystemObject
- Fire Support Officer, an Army or Marine Corps officer responsible for the planning and execution of Fires, and primary advisor to a maneuver commander on the application and integration of fires into the operation.
- Flash Shared Object a local shared cookie used in computing
- Floating Storage and Offloading, system, is an offshore oil and gas vessel
- Foreign Service Officer, government employee working for their country in a foreign country
- Forest Section Officer, an officer in the State Forest Services of the government of India
- Frank Skinner's Opinionated, British TV programme
- FreeSpace Open, an open-souce project to modernize the FreeSpace 2 Engine.
- Free-space optical communication, a technology that uses light to transmit data
- "F.S.O.", the first single from the Regurgitator album Tu-Plang
- Fulham Symphony Orchestra
- Full Scale Output test and measurement sensor
- Office of the Financial Secretary, Hong Kong