| Office of Instructional Services | Academic & Science |
| Oslo International School | Academic & Science |
| Office for Information Systems | Academic & Science |
| Office of International Students | Academic & Science |
| Oil States International | Business |
| O'Brien Investment Services | Business |
| Original Inner Sleeve | Business |
| OMNI Information Services | Business |
| Oakcrest Intermediate School | Community |
| Open Innovation Summit | Community |
| Oxford Intermediate School | Community |
| Conference on Office Information Systems | Community |
| Orinda Intermediate School | Community |
| Open Interconnected System | Computer Hardware |
| Open Interface Specification | Computing |
| Online Information System | Computing |
| Online Information Service | Computing |
| Object Oriented Input System | Computing |
| Overhead Information Symbol | Computing |
| Optimal Image Scale | General |
| Office for International Services | General |
| Ohio Intermodal Services | General |
| Operational Information Service | General |
| Ophthalmology Innovation Summits | General |
| Onboard Information System | General |
| Odometer Inspection Services | General |
| Office of International Services | General |
| Overnight Indexed Swap | General |
| Optical Image Stabilization | General |
| Office of International Studies | General |
| Oriented Input System | General |
| Outpatient Infusion Systems | General |
| Optical Image Stabilisation | General |
| Office Installation Services | General |
| Other Interpretations of Sentences | General |
| Office of Information Systems | General |
| Objective Interface Systems | General |
| Oregon Intervention System | General |
| Open Innovation Space | General |
| Overnight Index Swap | General |
| Operational Information system | Governmental |
| Organization Information Structure | Governmental |
| Office Information System | Governmental |
| Office of Information Services | Governmental |
| Officer-Involved Shooting | Governmental |
| Oriental Institute Seminars | Governmental |
| Oahu Information Service | Internet |
| Oh, I See! | Internet |
| Optical Intrinsic Signal | Medical |
| Only In Spain | Messaging |
| Officer Indoctrination School | Military and Defence |
| Operational Intercommunication System | Space Science |
| Orbiter Insertion Stage | Space Science |
| Optical Image Stabilizer | Space Science |
| Orbiter Instrumentation System | Space Science |
