| Acronym | Meaning |
| OTC | Open Tubular Column |
| OTC | Organic Toxic Compound |
| OTCEC | Open Tubular Capillary Electrochromatography |
| OTDR | Optical Time Domain Reflectometry |
| OTE | Optically Transparent Electrode |
| OTE | Oxygen Transfer Efficiency (of Aeration Diffusers) |
| OTEC | Ocean Thermal Energy Conversion |
| OTEC | Open Tubular Electrochromatography |
| OTEQ | One-dimensional Transport With Equilibria Calculations |
| OTFT | Organic Thin-film Transistor |
| OTIS | Oklahoma Telecommunications Interlibrary System |
| OTIS | One-dimensional Transport With Inflow And Storage |
| OTLC | Open Tubular Liquid Chromatography |
| OTS | Office Of Toxic Substances (USEPA) |
| OTTLE | Optically Transparent Thin-layer Electrode |
| OU | Odor Units |
| OUR | Oxygen Uptake Or Utilization Rate |
| OUST | Office Of Underground Storage Tanks |
| OVB | Orthogonalized Valence Bond Method |
| OVC | Optimized Valence Configuration |
| OVD | Optically Variable Device |
| OVD | Outside Vapor Deposition |
| OVFF | Orbital Valence Force Field |
| OVGF | Outer-valence Green Function (for Ionization-energy Calcns.) |
| OVMO | Occupied Valence-type Mol. Orbital |
| OVOC | Other Volatile Organic Compound |
| OVOS | Optimized Virtual Orbital Space Approximation (for Correlated Calcns.) |
| OW | Office Of Water |
| OWEC | Office Of Wastewater Enforcement And Compliance |
| OWG | Optical Waveguide |
| OWLS | Optical Waveguide Lightmode Spectroscopy |
| OWOW | Office Of Wetlands |
| OWPE | Office Waste Programs Enforcement |
| OWRS | Office Water Regulations And Standards (US EPA) |
| OWVP | Outgoing Wave Variational Principle |
| OZIPM | Ozone Isopleth Plotting With Optional Mechanisms |
| P-A TEST | Presence-absence Test |
| P-T-T-D | Pressure-temperature-time-deformation Path (for Metamorphic Rocks) |
| PA | Photoinduced Absorption |
| PA | Preliminary Assessment |
| PA-FTIR | Photoacoustic Fourier Transform Infrared Spectroscopy |
| PA/SI | Preliminary Assessment/Site Inspection |
| PAA | Photon Activation Analysis |
| PAA | Poly (acrylic Acid) |
| PAAP | Platelet Aggregation-associated Protein |
| PAB | Pollutant Assessment Branch (US EPA) |
| PAB | Particle Associated Bacteria |
| PAB | Passenger-side Air Bag |
| PABA | P-aminobenzoic Acid |
| PAC | P1-derived Artificial Chromosome |