| Acronym | Definition |
|---|
| OS | Operating System |
| OS | Oh Shoot (polite form) |
| OS | Oculus Sinister (Latin: Left Eye) |
| OS | Open Source |
| OS | Overseas |
| OS | Omaha Steaks (meat company) |
| OS | On-Site |
| OS | Ordnance Survey |
| OS | Online Services |
| OS | Old School |
| OS | One Side |
| OS | Old Style (or Old System; referring to Julian Calendars) |
| OS | Open Systems |
| OS | Old Style |
| OS | Out of Stock |
| OS | Outer Space |
| OS | Oversold (stock market) |
| OS | On Schedule |
| OS | Oversize |
| OS | Old Stuff :-) |
| OS | Office of the Secretary |
| OS | Off Screen (cinematography) |
| OS | Austrian Airlines (IATA airline code) |
| OS | Overall Survival |
| Os | Orioles (Baltimore baseball team) |
| OS | Office Support |
| OS | Oliver Stone (director) |
| OS | Outsourcing Services |
| OS | Osmium |
| OS | Out-of-Service |
| OS | Overshoot |
| OS | Over the Shoulder (cinematography) |
| os | Ossetian (language identifier; RFC 3066) |
| OS | Orlando Sentinel (Orlando, FL newspaper) |
| OS | Outlaw Star (action-anime) |
| OS | Only Son |
| OS | Owen Sound (City, Canada) |
| OS | Os Giken (automotive racing parts manufacturer) |
| OS | Old Spice (after shave brand) |
| OS | Off Stage |
| OS | Operational Security |
| OS | Open Sockets (Diablo II gaming) |
| OS | On Scene |
| OS | Output Status |
| OS | Oxygen Sensor |
| OS | Over Stock |
| OS | Observational Study |
| OS | Optical Scanning |
| OS | Operations Specialist (USN Rating) |
| OS | Over Speed |
| OS | Own Ship |
| OS | Ocean Springs (Mississippi) |
| OS | Onboard Software |
| OS | Orthopedic Surgeon |
| OS | Oberseminar |
| OS | Outstation |
| OS | Old Saxon |
| OS | Organizational Support |
| OS | Operserv (IRC Operator Server) |
| OS | Orbital Station |
| OS | Ordinary Seaman |
| OS | Office of Security |
| OS | Osijek, Croatia (license plate) |
| OS | Ordinary Seaman (Canadian naval rank) |
| OS | Order Statistic |
| OS | Operations Squadron |
| OS | Open Skies treaty |
| OS | Office Superintendent |
| OS | Oil Separator |
| OS | Outdoor Skills |
| OS | Ouvrier Spécialisé (French: semi-skilled worker) |
| OS | Off Scene |
| OS | Ocean Surveillance |
| OS | Obiettivi Specifici |
| OS | Office of Sustainability |
| OS | Operating Statement |
| OS | Operating Standard |
| OS | Office of Systems |
| OS | Opening Snap (mitral valve) |
| OS | Optical Sight |
| OS | Output Spy |
| OS | Ohr Somayach (school) |
| OS | Offensive Support |
| OS | Observation Squadron |
| OS | Orthopterists' Society |
| OS | Object Slice |
| OS | Operational Suitability |
| OS | Operating Strength |
| OS | Optical Silicon (semiconductor chip components) |
| OS | Organized Stalking |
| OS | Over Shield (Halo video game) |
| OS | Overtime Scheduled |
| OS | Oblates of Wisdom |
| OS | Operationalizing the Strategy |
| OS | Ogawa Shigeo (Engines) |
| OS | Otm Shank Jazz Quintet (band) |
| OS | Organic Signal |
| OS | Orienting Station (surveyed reference location for artillery position) |
| OS | Ordnance Specification |
| OS | Operational Superintendence (on overtime forms) |
| OS | Old Silcoatian (Silcoates School, Wakefield, UK) |