| Acronym | Meaning |
 | OPAL | Omni-Purpose Apparatus At LEP |
| OPAL | Optical Pumping Absorption Lidar |
| OPANA | Operational Version Of The Atm. Mesoscale Numerical Pollution Model |
| OPC | Optical Particle Counter |
| OPC | Ordinary Portland Cement |
| OPCW | Organization For Prohibition Of Chemical Weapons |
| OPDIN | Ocean Pollution Data And Information Network |
| OPE | Operator Product Expansion |
| OPE | Ozone Production Efficiency |
| OPEC | Organization Of Petroleum Exporting Countries |
| OPEE | Organic Phase Enzyme Electrode |
| OPG | Optical Parametric Generator |
| OPGF | One-particle Green Function |
| OPHF | Orbital-polarized Hartree-Fock |
| OPIA | Optical Precision Instrument Associates |
| OPIDP | Organophosphate Induced Delayed Polyneuropathy |
| OPIM | Other Potential Infectious Material |
| OPIT | Oxide Powder In Tube |
| OPLC | Overpressured Layer Chromatography |
| OPM | Open Path Monitors (air Pollution Sensors) |
| OPM | Optimized Potential Model |
| OPO | Optical Parametric Oscillator |
| OPP | Office Of Pesticide Programs |
| OPPE | Office Of Policy |
| OPPIS | Optically Pumped Polarized Ion Source |
| OPPT | Office Of Pollution Prevention And Toxics (US EPA) |
| OPPTS | Office For Prevention |
| OPSE | Optically Pumped Stimulated Emission |
| OPTS | Office Of Pesticides And Toxic Substances (USEPA) |
| OPW | Orthogonalized Plane Wave |
| OR | Odds Ratio |
| ORAC | Organic Reaction Accessed By Computer |
| ORAC | Oxygen Radical Absorbance Capacity |
| ORACLE | Ozone Research Using Advanced Cooperative Lidar Experiment |
| ORAM | Optical Random Access Memory |
| ORAQI | Oak Ridge Air Quality Index |
| ORAU | Oxford Radiocarbon Accelerator Unit (University Of Oxford) |
| ORBIT | On-line Retrieval Of Bibliographic Information Timesharing |
| ORC | Origin Recognition Complex |
| ORCA | Optical Resonance Cavity Analyzer |
| ORD | Office Of Research And Development (USEPA) |
| ORD | Optical Rotatory Dispersion |
| ORD BBS | ORD Electronic Bulletin Board System |
| ORF | Open Reading Frame (DNA Sequence) |
| ORFEUS | Orbiting Retrievable Far And Extreme Ultraviolet Spectrometers |
| ORGDL | Oak Ridge Gaseous Diffusion Lab. |
| ORI | Octane Requirement Increase |
| ORM | Other Regulated Material (DOT) |
| ORNL | Oak Ridge National Laboratory |
| ORO | Office Of Regional Operations |
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