National Laboratory ORR – (i) Oak Ridge Reservation (U.S. DOE) – Office of Rail Regulation (UK government) – Office of Refugee Resettlement – (CIA) Office of Research and Reports – Operational Readiness Rate – Operational Readiness Review ORSA – Oxacillin-resistant Staphylococcus aureus – Operations Research Society of America OS OS – (s) Operating System os – (s) Ossetic language (ISO 639-1 code) Os – (s) Osmium OSA – (i) Order of St. Augustine OSCE – (i) Organization for Security and Co-operation 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 OSGeo - (a) Open Source Geospatial Foundation (OSGeo) 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 OSIRIS (a) Optical, Spectroscopic, and Infrared Remote Imaging System (astronomical system on the Rosetta spacecraft) Optical System for Imaging and low Resolution Integrated Spectroscopy (astronomical system on the land-based Gran Telescopio Canarias) OSM – (i) One Saturday Morning OSMF - (a) OpenStreetMap Foundation 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 OT OTA – Occupational Therapy Assistant, (i) Office of Technology Assessment – (a/i) Orthodontic Technicians Association OTAC – (i) Orthodontic Technicians Association Conference – (i) Orthodontic Technicians Association Council OTB – (i) Off The Ball – (i) Offtrack Betting – (p) OneSAF Testbed (a descendant of ModSAF) OTC – (i) Over-The-Counter (drugs) OTD – (i) Off the derech (referring to an Orthodox Jew who has stopped practicing the tenets of his or her faith) OTEA – (i) (U.S. Army) Operational Test Evaluation Agency OTI – (i) Office of Transition Initiatives (United States Agency for International Development) OTO – (i) Ordo Templi Orientis (Latin, "Order of the Temple of