| Otto-Suhr-Institut | Academic & Science |
| Oxford Sciences Innovation | Academic & Science |
| Office of Student Involvement | Academic & Science |
| Office Of Special Investigation | Academic & Science |
| Osijek | Airport Code |
| Orchestra della Svizzera Italiana | Arts |
| Objective Systems Integrators | Business |
| Officine Stampaggi Industriali | Business |
| Ohio Scientific Instruments | Business |
| Office of Superintendent of Insurance | Business |
| Ocean Specialists Inc | Business |
| Orthopedic Surgeons Inc | Business |
| Sealants Inc | Business |
| Officine Stampaggi Industriale | Business |
| Origin Systems, Incorporated | Business |
| Operating System Intelligence | Business |
| Origin Systems Inc | Business |
| OSI Geospatial Incorporated (de-listed) | Business |
| Office Solutions Inc | Business |
| Systems Inc | Business |
| Orchestra of Strategic Influence | Chemistry |
| Ordinary Stuffed Isotopes | Chemistry |
| Open Source Institute | Community |
| Open Switching Interval | Computer and Networking |
| Open System Interconnection | Computer and Networking |
| Optimal System Integration | Computer Hardware |
| Overflow Stack Immediate | Computing |
| Open Source India | Computing |
| Open System Interface | Computing |
| Open Society Institute | Computing |
| Operating System Instance | General |
| Oregon Screen Impressions | General |
| Open Scholarship Initiative | General |
| Only Some Interest | General |
| Online Search Interest | General |
| Option Symbology Initiative | General |
| Office of Strategic Initiatives | General |
| Open Systematic Information | General |
| Officina Stampaggi Industriali | General |
| Oil Stability Index | General |
| Office Of Scientific Information | General |
| Open System Inter | General |
| The Overall Satisfaction Index | General |
| Open Solver Interface | General |
| Open Systems International | General |
| Ocean Star International | General |
| Open Systems Interconnect | General |
| Office of Systems Integration | General |
| Other Significant Information | General |
| Office Of Strategic Influence | General |
| Open Source Indicators | General |
| Office of Scientific Intelligence | Governmental |
| Office of Scientific Integrity | Governmental |
| Office of Scientific Investigation | Governmental |
| Office Of Supernatural Investigation | Governmental |
| A Organization For Standards Institute | Governmental |
| Office of Special Investigations | Governmental |
| Open Space Institute | Governmental |
| Open Source Initiative | Information Technology |
| Open Systems Interconnection | Information Technology |
| Operation Serve International | International |
| διασύνδεση ανοικτών συστηάτων | International |
| Only Safe Internet | Internet |
| Orthopaedic Specialty Institute | Medical |
| Office of Strategic Intelligence | Military and Defence |
| Operational Subsystem Interface | Military and Defence |
| Ordnance Survey Ireland | Regional |
| Osijek Airport | Transport & Travel |