General

• OSI Pharmaceuticals,an American pharmaceutical company
• OSI Restaurant Partners, the restaurant and entertainment group that includes Outback Steakhouse
• OSI Systems a company that manufactures security scanners and medical equipment based in California
• Objective Systems Integrators a company providing Operational Support Systems
• Olomouc Summer Institute, an educational program for high-school students
• Ontario Swine Improvement, a pig breeding company based in Ontario, Canada
• Open Space Institute, an organization that seeks to preserve scenic, natural and historic landscapes
• Open Society Institute, a private charitable foundation established by George Soros to promote open societies around the world
• Ordnance Survey of Ireland (Suirbhéireacht Ordanáis Éireann)
• Otto-Suhr-Institut, the political-science institute of the Free University of Berlin

Government

• Office of Scientific Intelligence, former name of a department of the Central Intelligence Agency now called the Directorate of Science and Technology
• Office of Strategic Influence, a short-lived former American government agency created in 2001, part of the Department of Defense and focused on psychological warfare
• Office of Special Investigations, a name shared by several government agencies
• Air Force Office of Special Investigations
• Office of Special Investigations (Government Accountability Office)
• Office of Special Investigations (United States Department of Justice)
• Office of Scientific Integrity, now United States Office of Research Integrity

Technology

• Open Systems Interconnection, a joint ISO and ITU-T standard for computer networks and communication protocols
• OSI model, a layered description for communications and computer network protocol design
• Open Source Initiative, an organization dedicated to promoting open source software
• Ohio Scientific Inc., a microcomputer manufacturer from the late 1970s to the early 1980s
• Open Switching Interval, a type of disconnect supervision
• On-Site Inspection is an advanced measure technique combining several methods - especially used in order to detect nuclear tests.

Entertainment

• OSI (band), an American progressive rock supergroup named after the former Office of Strategic Influence government agency
• Origin Systems, a former United States computer game developer
• Office of Strategic Influence (album), the 2003 debut album by the band OSI

Fiction

• "OSI", a fictional United States government agency in The Six Million Dollar Man and Bionic Woman television programs
• "Office of Secret Intelligence", a fictional United States government secret agency in the animated series The Venture Bros.

People

• Osi Umenyiora, defensive end for the New York Giants.