| Acronym | Meaning |
| ORP | Oxidation-reduction Potential |
| ORPA | Optimized Random Phase Approximation |
| ORPI | Off-resonance Proton Irradiation |
| ORPP | Optimized Relativistic Pseudopotential |
| ORS | Odds Ratios |
| ORSTB | Overlap-reduced Semiempirical Tight-binding |
| ORU | In Orbit Replaceable Unit (spacecraft) |
| ORV | Optical Retardation Velocity |
| OS | Overlapping Spheres (method) |
| OSA | Optical Society Of America |
| OSAC | Organic Structures Accessed By Computer |
| OSAILS | Optical Sensor Array And Integrated Light Source |
| OSBG | Optimized Support For Biological Growth |
| OSBP | Oxysterol-binding Protein |
| OSC | On Site Coordinator |
| OSC | Oxygen Storage Capacity |
| OSCAR | Oxidn. And Scavenging Characteristics Of April Rains |
| OSCC | Oral Squamous Cell Carcinoma |
| OSCCM | Open-shell Coupled-cluster Method (in Electronic Property Calcn.) |
| OSEE | Optically Stimulated Electron Emission |
| OSF | Odor Safety Factor |
| OSHA | Occupational Safety And Health Act Or Administration |
| OSHRC | Occupational Safety And Health Review Commission |
| OSIRIS | Optical Spectrograph And IR Imaging System |
| OSIRIS | Oxford Supersensitive Injector For Radioactive Isotope Separation |
| OSL | Optically Stimulated Luminescence |
| OSM | US Office Of Surface Mining Reclamation And Enforcement |
| OSM | Overlapping Sphere Model |
| OSPARCOM | Oslo And Paris Commission [Environmental Regulations For The European Community] |
| OSR | Office Of Science And Research |
| OSRD | Office Of Scientific Research And Development |
| OSRHF | Open-shell Restricted-Hartree-Fock |
| OSRSCF | Open-shell Restricted SCF |
| OSSA | Office Of Space Science And Applications (NASA) |
| OSSE | Oriented Scintillation Spectrometer Experiment |
| OSTE | One-step Temper Embrittlement |
| OSTI | Office Of Scientific And Technical Information |
| OSTL | Optically Stimulated Thermoluminescence |
| OSTLS | Optical Saturation Thermal Lens Spectrometry |
| OSTP | Office Of Science And Technology Policy |
| OSW | Office Of Solid Waste (US EPA) |
| OSW | US Office Of Saline Water |
| OSWASV | Osteryoung Square-wave Anodic Stripping Voltammetry |
| OSWER | Office Of Solid Waste And Emergency Response (USEPA) |
| OSWER BBS | OSWER Electronic Bulletin Board System (see CLU-IN) |
| OSWSV | Osteryoung Square Wave Stripping Voltammetry |
| OSWV | Osteryoung Square Wave Voltammetry |
| OTA | Office Of Technology Assessment |
| OTAG | Ozone Transport Assessment Group |
| OTC | Ozone Transport Commission |