| Acronym | Meaning |
| NWRI | National Water Research Institute (Burlington |
| NWS | National Weather Service |
| NWSIA | National Water Supply Improvement Association |
| NWWA | National Water Well Association |
| NWWA | Northwest Water Authority (UK) |
| NYSAS | New York Summer Aerosol Study |
| NYSDEC | New York State Department Of Environmental Conservation |
| NYSERDA | New York State Energy Research And Development Authority |
| NZRPA | Normalized Zero-range Potential Approxn. |
| O/E | Observed Number/expected Number Ratio = Relative Risk |
| OA | Office Of The Administrator |
| OAM | Optical Absorption Microscope |
| OAO | Orthogonalized Atomic Orbital |
| OAP | Optical Atom Probe |
| OAPP | Oxidized Atactic Polypropylene |
| OAQPS | Office Of Air Quality Planning And Standards |
| OAQPSTTN | Office Of Air Quality Planning And Standards Technology Transfer Network |
| OAR | Office Of Air And Radiation (USEPA) |
| OARM | Office Of Administration And Resources Management (USEPA) |
| OASCR | Office Of Advanced Scientific Computing Research |
| OASIS | Ocean And Atmospheric Sounding Interferometer System |
| OASLM | Optically Addressed Spatial Light Modulator |
| OAW | Oxyacetylene Welding |
| OB/OD | Open Burn/open Detonation |
| OBD | Optical Beam Deflection |
| OBD | On-board Diagnosis |
| OBDD | Ordered Bicontinuous Double-diamond |
| OBE | Optical Bloch Equation |
| OBIC | Optical Beam Induced Current |
| OBK | Oppenheimer-Brinkman-Kramers (approxn. In Atom-ion Electron-exchange Calcns.) |
| OBK2 | Second-order Oppenheimer-Brinkman-Kramers |
| OBK3 | Third-order Oppenheimer-Brinkman-Kramers |
| OBS-MCSC | Optimized Basis Set-multiconfiguration Spin Coupled |
| OC | Office Of The Comptroller |
| OC | Oxygen Consumed |
| OCAMS | Orbital Correspondence Analysis In Maximum Symmetry |
| OCAP | Open-channel Air Preheater |
| OCBSE | Orthogonality Constrained Basis Set Expansion Prodedure |
| OCC | Organochlorinated Compound |
| OCCSD | Open-shell Coupled-cluster (with) Single (and) Double (excitations) |
| OCD | Offshore And Coastal Dispersion (modeling Of Air Quality) |
| OCD | Overlapping Charge Density (model In Electronic Structure) |
| OCE | One-center Expansion (SCF MO Method) |
| OCE | Overall Control Efficiency |
| OCE-DF | One-center Expansion Dirac-Fock |
| OCEC | Open-channel Electrochromatography |
| OCEE | Overlap Corrected Electronegativity Equalization |
| OCEFT | Office Of Criminal Enforcement |
| OCEPA | Office Of Communications |
| OCH | Ochkur (approxn. In Electron-atom Scattering Calcns.) |