| Acronym | Definition |
|---|
| FSO | Free Space Optics |
| FSO | Free Space Optical |
| FSO | File System Object (Microsoft) |
| FSO | Faculté des Sciences Oujda (French: Oujda Science Faculty; Oujda, Morocco) |
| FSO | Facilities Security Officer |
| FSO | Fire Safety Order (regulation) |
| FSO | Foreign Service Officer |
| FSO | Fabryka Samochodow Osobowych |
| FSO | File System Object |
| FSO | Field Service Organization |
| FSO | Field Selector Outside |
| FSO | Floating Storage and Offloading (supertankers usually used for oil shipping) |
| FSO | Federal Statistical Office (Germany) |
| FSO | Full Scale Output |
| FSO | Fire Support Officer |
| FSO | Freesmartphone |
| FSO | Financial Service Organization |
| FSO | Flag Service Organization (Scientology) |
| FSO | Fabryka Samochodow Osobowych (Poland passenger automobile factory) |
| FSO | Fund for Special Operations |
| FSO | Family Support Organization |
| FSO | Floating Storage Offloading |
| FSO | Flotilla Staff Officer |
| FSO | File Scripting Object (computing) |
| FSO | Field Security Operations |
| FSO | Financial Services Officer |
| FSO | Full Spectrum Operations |
| FSO | Flight Safety Officer |
| FSO | Food Service Officer |
| FSO | Field Security Officer (United Nations) |
| FSO | Flagstaff Symphony Orchestra (Flagstaff, Arizona) |
| FSO | Fox Sports Ohio |
| FSO | Full Sail University Online (Winter Park, FL) |
| FSO | Field Security Office |
| FSO | Flint Symphony Orchestra (Flint, Michigan) |
| FSO | First Star Online (game) |
| FSO | Food Stamp Outreach (various organizations) |
| FSO | Funeral Service Operative (undertaker) |
| FSO | Forum Support Organisation |
| FSO | Former Significant Other |
| FSO | Financial Sanctions Officer |
| FSO | Freinage du Sud-Ouest (French: Braking Southwest) |
| FSO | For Services Of |
| FSO | Friday Shout Out |
| FSO | Financial Services of Ontario |
| FSO | Field Service Officer |
| FSO | Fender Shaped Object (musical instruments) |
| FSO | Facilities Service Office (USPS) |
| FSO | Federalnaya Sluzhba Okhrany (Russian: Federal Security Organization; successor of GUO and SBP) |
| FSO | Field Service Office |
| FSO | Fail-Safe Open |
| FSO | Field Sales Office |
| FSO | Family Service Ontario |
| FSO | Fleet Support Operations |
| FSO | Fill Station Operator |
| FSO | Flight Surgeon's Office |
| FSO | Full Service Office |
| FSO | Fleet Support Officer |
| FSO | Fuel Service Option (automobile rental) |
| FSO | Firewall Security Officer |
| FSO | Flight System Operations (US NASA) |
| FSO | Fuel Supply Office |
| FSO | Fédération Suisse des Osthéopathes (French: Swiss Federation of Osteopaths) |
| FSO | Facilities and Services for Older Persons |
| FSO | Facility Support Operator |
| FSO | Facility Security Officer/Supervisor |
| FSO | Field Service Orders (accounting software) |
| FSO | Freshman Scholarship Options |
| FSO | Familles Sang et Or (French: Blood and Gold Families; Belgium) |
| FSO | Factory Shipping Order |
| FSO | For Spares Order |
| FSO | Form, Space, and Order |
| FSO | Free Standing Option (new vehicle option that is not sold as part of a package) |
| FSO | Fixed Site of Operation |
| FSO | Force Structure Option |