| Acronym | Definition |
|---|
| SO | Significant Other |
| SO | Sistema Operativo (Italian: Operating System) |
| SO | Special Order |
| SO | Sign Off |
| SO | Second Order |
| SO | Sales Order |
| SO | Server Options |
| SO | Service Order |
| SO | Standard Oil |
| SO | Sold Out |
| SO | Student Organization |
| SO | Safety Office |
| SO | Strike Out |
| SO | Standing Order |
| SO | Special Operations |
| SO | Southern Company (stock symbol) |
| SO | Security Officer |
| SO | Senior Officer |
| SO | Safety Officer |
| SO | System Operator |
| SO | Star Ocean (video game) |
| SO | Sims Online (game) |
| SO | Symphony Orchestra |
| SO | Staff Officer |
| SO | Special Olympics |
| SO | Somalia |
| SO | Shout Out (Internet slang) |
| SO | Sheriff's Office |
| SO | Small-Outline |
| SO | Stuck Open |
| SO | Stack Overflow |
| SO | Second Officer |
| SO | Sub-Optimal |
| SO | Shared Object (UNIX) |
| SO | Southampton (UK postal code) |
| SO | Strategic Objective |
| SO | Service Object |
| SO | Systems Operations |
| SO | System Owner (US NIST) |
| SO | Sistema Operacional (Brazil: Operating System) |
| SO | Shutout |
| SO | Serial Output |
| SO | Shaquille O'neal (NBA Player) |
| SO | Solothurn (Soleure; Suisse Canton) |
| SO | System Overload |
| SO | Strung Out (band) |
| SO | School's Out |
| SO | Sud-Ouest (French: south west) |
| So | Siehe Oben (German: see above) |
| SO | Science Olympiad |
| SO | Station Officer |
| SO | Send Only |
| SO | Staff Officer (commissioned Police Officer attached to Inspector-General of Police) |
| SO | Shift-Out |
| SO | Spatial Orientation |
| SO | Southern Oscillation |
| SO | Socket Outlet (British, electrical) |
| SO | South/Southern |
| SO | Strategic Outsourcing (IBM) |
| SO | Sales Out |
| SO | Scottish Office |
| SO | Supply Officer |
| SO | Sud-Ovest (Italian: Southwest) |
| SO | Station Operator |
| SO | Supervisor's Office (USDA) |
| SO | Successful Offeror |
| SO | System Outage |
| SO | Superior Oblique (eye muscle) |
| SO | Seller's Option |
| SO | Sea Organization (Scientology) |
| SO | Switching Office |
| SO | Steward Observatory (University of Arizona) |
| SO | Specialist Operations |
| SO | Sensor Operator |
| SO | Southern Airways (airline) |
| SO | Special Orthogonal (mathematics; group theory) |
| SO | Safety Observer |
| SO | Solid Optical |
| SO | Slow Oxidative |
| SO | Sondrio, Lombardia (Italian province) |
| SO | Sans Opinion (French : No Preference) |
| SO | System Overheat |
| SO | Stewardship Ontario (Canada) |
| SO | Semantic Object |
| SO | Sector Outperform (investing) |
| SO | Service Observe |
| SO | Division Staff Officer |
| SO | Show of Oil (drilling reports) |
| SO | Surface Operator (MFD) |
| SO | Semi-Orthogonal |
| SO | Stockage Objective(s) |
| SO | Source of Orgasm |
| SO | Synchronous Oscillator |
| SO | Supporting Organization of ICANN |
| SO | Service Overseer (Jehovah's Witnesses) |
| SO | Schriftelijke Overhoring |
| SO | Solicitation Officer |
| SO | Saneringsonderzoek (Dutch: soil remediation research) |
| SO | Step Off Ministry |
| SO | Shipment/Shipping Order |
| SO | Seige Onager (Age of Kings game) |
| SO | Südost/Südosten (German: South-East) |