| Acronym | Meaning |
| OIRM | Office Of Information Resources Management |
| OIT | Oxidative Induction Time |
| OKE | Optical Kerr Effect |
| OLCAO | Orthogonalized Linear Combination Of Atomic Orbitals |
| OLDA | Ordinary Linear Discriminant Analysis |
| OLED | Organic Light-emitting Device |
| OLF | Overland Flow |
| OLGA | On-Line Gas Chemistry Apparatus |
| OLMO | Orthonormal Localized Mol. Orbital |
| OLO | On-line Optimization |
| OLPA | Orbital Local Plasma Approxn. (in Stopping Power Calcn. For Ions By Films) |
| OLPGAA | On-line Prompt Gamma-ray Activation Analysis |
| OLR | Organic Loading Rate |
| OLS | Ordinary Least Squares |
| OLS | Online Library System |
| OLSR | Ordinary Least Squares Regression |
| OLT | Orthotopic Liver Transplantation |
| OMB | Office Of Management And Budget |
| OMBD | Organic Molecular Beam Deposition |
| OMBE | Organic Molecular Beam Epitaxy |
| OMCVD | Organometallic Chemical Vapor Deposition |
| OMEGA | Method In Molecular Orbital Calculations |
| OMEX | Ocean Margin Exchange |
| OMHCE | Organic Material Hydrocarbon Equivalent |
| OMIM | Online Mendelian Inheritance In Man |
| OMMSQA | Office Of Modeling |
| OMNIS | Observatory For Multiflavor Neutrinos From Supernovae |
| OMNMHCE | Organic Material Nonmethane Hydrocarbon Equivalent |
| OMO | Orthogonalized Magnetic Orbital |
| OMO | Orthonormal Molecular Orbital |
| OMP | Optimized Model Potential |
| OMP | Outer Membrane Protein |
| OMPW | Orthogonalized Modulated Plane Wave (wave Function) |
| OMS | Octahedral Molecular Sieve |
| OMTP | Overlap-multipole-expansion Procedure |
| OMVPE | Organometallic Vapor-phase Epitaxy |
| OMZ | Oxygen Min. Zone |
| ONBI | Oligonucleotide Bridge Intercalator |
| ONCV | Orthonormality Constrained Variation (quantum Method) |
| ONDOR | Optical Nuclear Double Resonance |
| ONP | Old Newspaper |
| ONR | Office Of Naval Research |
| ONWI | Office Of Nuclear Waste Isolation |
| OOA | Oxygenated Organic Aerosol |
| OOS | Optical-oscillator Strength |
| OP | Oceanic Plagiogranites |
| OP | Overlap Population |
| OP-AMP | Operational Amplifier |
| OPA | Optical Parametric Amplifier |
| OPACA | Organotin Paint Control Act Of 1988 (US) |