| O | Oxygen |
| O-D | Zero Dimensional |
| OBA | Object Behavior Analysis |
| OBEM | Object-Based Equipment Model |
| OBIC | Optical Beam-induced Current |
| OBL | Object-based Language |
| OC | Open Cassette |
| OCR | Optical Character Recognition |
| OD | Outside Diameter |
| OD | Optical Density |
| ODBC | Open Database Connectivity |
| ODC | Oxygen-doped Carbide |
| ODE | Ordinary Differential Equation |
| ODP | Ozone Depletion Potential |
| ODS | Ozone-depleting Substances |
| ODU | Octapole Drive Unit |
| OEBR | Optical Edge Bead Removal |
| OEE | Overall Equipment Effectiveness |
| OEL | Occupational Exposure Limit |
| OEM | Original Equipment Manufacturer |
| OES | Optical Emission Spectroscopy |
| OFA | Oil-free Air |
| OFE | Overall Factory Effectiveness |
| OHS | Overhead Hoist Shuttle |
| OHT | Overhead Transport |
| OHT | Overhead Hoist Transport |
| OHV | Overhead Vehicle |
| OHVC | Overhead Hoist Vehicle Controller |
| OID | Object Interaction Diagram |
| OID | Object Identification |
| OIM | Orientation Imaging Microscopy |
| OL | Objective Lens |
| OL | Overlay |
| OLB | Outer Lead Bond |
| OLE | Object Linking And Embedding |
| OLE | Object Linking Environment |
| OLS | Ordinary Least Squares |
| OM | Operational Modeling |
| OM | Optical Microscopy |
| OMA | Object Management Architecture |
| OMS | Optical Mass Spectroscopy |
| OMT | Object Modeling Technique |
| OO | Object-oriented |
| OOA | Object-oriented Analysis |
| OOD | Object-oriented Design |
| OODB | Object-oriented Database |
| OODBMS | Object-oriented Database Management System |
| OOP | Object-oriented Programming |
| OOPS | Object-oriented Programming System |
| OPC | Optical Particle Counter |
| OPC | Optical Proximity Correction |
| OPD | Out-of-plane Distortion |
| OPL | Optical Pathlength |
| OPL | Optical Planarization Layer |
| OPS | Optically Pumped Semiconductor |
| ORB | Object Request Broker |
| ORM | Other Regulated Material |
| ORNL | Oak Ridge National Laboratory |
| ORP | Oxidation-reduction Potential |
| Os | Osmium |
| OS | Operating System |
| OSD | Organic Spin-on Dielectric |
| OSF | Open Systems Foundation |
| OSF | Oxidation-induced Stacking Fault |
| OSG | Organosilicate Glass |
| OSI | Open System Interconnection |
| OSRM | Office Of Standard Reference Materials |
| OSS | Object Services Standard |
| OSV | Offset Scan Voting |
| OTF | Optical Transfer Function |
| OVL | Overlay |
| Ox | Oxide |