OS
- OS – (s) Operating System
- os – (s) Ossetic language (ISO 639-1 code)
- Os – (s) Osmium
- OSA – (i) Order of St. Augustine
- OSCE – (i) Organisation for Security and Cooperation in Europe
- OSD – (i) U.S. Office of the Secretary of Defense – (i) On-screen Display
- OSDL – (i) Open Source Development Labs
- OSE – (a) Own Separate Entrance
- OSERS – (a) U.S. Department of Education Office of Special Education and Rehabilitative Services
- OSETI – (a) Optical SETI also Optical SETI
- OSHA – (a) Occupational Safety and Health Administration
- OSI – (i) Office of Strategic Influence – Ontario Swine Improvement – Open Source Initiative – Open Systems Interconnection
- OSIA – (i) (U.S.) On-Site Inspection Agency
- OSL – (i) Open Source License – Optically Stimulated Luminescence – Orbiting Solar Laboratory
- OSM – (i) One Saturday Morning
- OSOTPBOIAS – (i) One Sec, On The Phone, Be Off In A Sec
- OSPA – (a) Operations Security Professional's Association site of OSPA
- OSPA – (i) Oregon School Psychologists Association
- OSRH – (i) Oružane Snage Republike Hrvatske (en: Armed forces of the Republic of Croatia)
- oss – (s) Ossetic language (ISO 639-2 code)
- OSS – (i) Office of Strategic Services (1942–1945, forerunner of CIA)
- OSU – (i) Ohio State University – Oklahoma State University
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