| Oral Proficiency Interview | Academic & Science |
| Other Peoples' Interviews | Academic & Science |
| Offline Processing and Integration | Academic & Science |
| Open Payment Initiative | Accounts and Finance |
| Oenpelli | Airport Code |
| Over the Phone Interpreter | Business |
| Opening Package Inspection | Business |
| Operating Performance Index | Business |
| Office Products Inc | Business |
| Overall Performance Incentive | Business |
| Optical Phusion Inc | Business |
| Office Products International | Business |
| Occupational Performance Issue | Business |
| Ohio Penal Industries | Business |
| Online Product Information | Business |
| Ounce of Prevention Insurance | Business |
| Orthodontal Products Incorporated | Business |
| Outsource Partners International | Business |
| Odontorium Products Inc | Business |
| Office of Public Instruction | Community |
| Open Press Interface | Community |
| Open Prepress Interface | Computing |
| Open Postscript Interface | Computing |
| Open Peripheral Interface | Computing |
| Oracle Programmatic Interface | Computing |
| On Package Interconnect | Computing |
| Open Platform Initiative | Computing |
| Open Platform Interface | Computing |
| Optional Procedure Indicator | General |
| Online Placement Instrument | General |
| Olivia Pope Investigations | General |
| Overall Pavement Index | General |
| One Piece Integration | General |
| Open Perimetry Interface | General |
| Omnibus Personality Inventory | General |
| Organic Performance Index | General |
| Office of Primary Interest | General |
| Office Properties Income | General |
| Organizational Performance Index | General |
| Office of Public Integrity | General |
| Oregon Project Independence | General |
| Our Private Interhouse | General |
| Oxygen Percentage Indicator | General |
| Operations Procedure Instruction | General |
| Optimum Performance Institute | Governmental |
| Orientation Professionals Institute | Governmental |
| Office of Police Integrity | Governmental |
| Office of Performance Improvement | Governmental |
| Office of Planning and Informatics | Governmental |
| Oposición de Izquierda | International |
| Opioid Prescription Intervention | Medical |
| Obesity Prevention Initiative | Medical |
| Opiates | Medical |
| Officer of Principal Interest | Military and Defence |
| Oral Presentation Initiative | Military and Defence |
| Oenpelli, Northern Territory, Australia | Regional |
| Orbiter Payload Interrogator | Space Science |