| O | Oxygen |
| OA | Organic Acid |
| OAAF | Open Advanced Authoring Format |
| OAC | Optical Absorption Coefficient |
| OASIS | Organization for the Advancement of Structured Information Standards |
| OBA | Object Behavior Analysis |
| OBEM | Object-Based Equipment Model |
| OBIC | Optical Beam Induced Current |
| OBL | Object-Based Language |
| OBSAI | Open Base Station Architecture Initiative |
| OC | Open Collector; Open Circuit |
| OCAP | OpenCable Application Platform |
| OCR | Optical Character Recognition |
| OCSP | Online Certificate Status Protocol |
| OCXO | Oven Controlled Crystal Oscillator |
| OD | Outside Diameter |
| ODBC | Open DataBase Connectivity |
| ODEND | OR Optical Detected ENDOR |
| ODEPR | Optical Detected EPR |
| ODI | Open Data-link Interface |
| ODMII | Optical Detected Microwave induced Impact Ionization |
| ODMR | Optical Detected Magnetic Resonance |
| ODP | Ozone Depletion Potential |
| ODR | Oscillating Disk Rheometer |
| ODRL | Open Digital Rights Language |
| ODS | Ozone-Depleting Substances |
| OEBR | Optical Edge Bead Removal |
| OEE | Overall Equipment Effectiveness |
| OEIC | OptoElectronic IC |
| OEM | Original Equipment Manufacturer; Office of Emergency Management |
| OES | Optical Emission Spectroscopy |
| OFDM | Orthogonal Frequency Division Multiplexing |
| OFHC | Oxygen-Free High-Conductivity Copper |
| OFS | Open File System |
| OI | Opto Isolator |
| OID | Object Interaction Diagram; Object IDentification |
| OIL | Ontology Interface Layer |
| OIM | Orientational Imaging Microscopy |
| OL | Overlay |
| OLB | Outer Lead Bond |
| OLE | Object Linking and Embedding |
| OLED | Organic Light Emitting Diode |
| OLS | Ordinary Least Squares |
| OM | Operational Modeling; Optical Microscopy; OrganoMetallic |
| OMA | Object Management Architecture; Open Mobile Alliance |
| OMG | Object Management Group |
| OMS | Optical Mass Spectroscopy |
| OMT | Object Modeling Technique; Ortho-Mode Transducer |
| OMVPE | Organometallic VPE |
| ONE | Open Network Environment (Sun) |
| ONP | Optical Nuclear Polarization |
| OO | Object-Oriented |
| OOA | Object-Oriented Analysis |
| OOD | Object-Oriented Design |
| OODB | Object-Oriented Data Base |
| OODBMS | Object-Oriented Data Base Management System |
| OOP | Object Oriented Programming |
| OOPS | Object-Oriented Programming System |
| OPC | Optical Particle Counter; Optical Proximity Correction |
| OPL | Optically Pumped Laser |
| OPSE | Optically Pumped Stimulated Emission |
| OPW | Orthogonalized Plane Wave |
| ORB | Object Request Broker |
| ORM | Other Regulated Material |
| ORNL | Oak Ridge National Laboratory |
| OS | operating System |
| OSC | Organic Solderability Coating |
| OSCI | Online Services Computer Interface |
| OSD | On Screen Display |
| OSDH | Optical Shubnikhov-de Haas effect |
| OSF | Open Systems Foundation |
| OSHA | Occupational Safety Hazards Act |
| OSI | Open System Interconnection |
| OSP | Organic Solderability Preservative |
| OSRM | Office of Standard Reference Materials |
| OSS | Object Services Standard; Operation Support System |
| OSV | Offset Scan Voting |
| OTOI | 3rd order Output Intercept |
| OTP | One Time Programmable |
| OTPROM | One Time Programmable ROM |
| OVD | Outside Vapor Deposition |
| OVL | Overlay |
| O-D | Zero dimensional |
| Oe | Oersted (magnetic intensity unit) |
| OpAmp | Operational Amplifier |
| OR | Logic term - OR |
| OX | OXide |
