| O/U | Over/Under(arrangement of double barrels (shotgun)) |
| O&P | Ova and Parasites |
| OB/Gyn | Obstetrics/Gynecology |
| OC | On centers (expressing distance between parts, for instance) |
| OCAP | Out-of-Control Action Plan (quality assurance) |
| OCAW | Oil, Chemical & Atomic Workers (International Union) |
| OCBA | o-Chlorobenzoic acid |
| OCBC | o-Chlorobenzoyl chloride |
| OCBN | o-Chlorobenzonitrile |
| OCHS | Hydroxycorticosteroids |
| OCPA | o-Chlorophenylacetic acid |
| OCSC | Optoelectronic Computing Systems Center (U of Colo. and Colo State U.) |
| OCT | o-Chlorotoluene |
| OCTC | o-Chlorobenzotrichloride |
| OD | Optical Density (= Absorbance in chemical spectrophotometry) |
| OD | OverDose |
| OD | Ocula dextra |
| OD | Olive Drab (color commonly used for US Army equipment and clothing) |
| OD | Outside Diameter |
| ODP | Ozone Depleting[damaging/degrading] Potential |
| OECD | Organization for Economic Cooperation and Development3 |
| OEL | Operational exposure limit |
| OEM | Original Equipment Manufacturer |
| OES | Optical emission spectrometry |
| OFC | Optical Fiber Conference |
| OGTT | Oral Glucose Tolerance Test |
| OHA | Oral hypoglycemic agent |
| OHL | Overhung Load |
| OHSMS | Occupational Health and Safety Management System(s) |
| OIT | Office of Industrial Technologies(US Dept. of Energy) |
| OIT | Oxidation Induction Time |
| OITDA | Optoelectronic Industry and Technology Development Ass'n. (Japan) |
| OLE | Object Linking and Embedding (Microsoft Windows feature) |
| OMA | Optical Multichannel Analyzer |
| OMB | Office of Management and Budget (USA) |
| OMPA | Octamethylpyrophosphoramide |
| OMS | Organisation Mondiale de la Santé |
| ONPG | o-nitrophenyl-beta-D-galactosidase |
| ONR | Office of Naval Research (USA) |
| OOS | Out of specification |
| OPAO | Open Air-Over (electric motor) |
| OPCW | Organization for the Prohibition of Chemical Weapons |
| OPG | Oculoplethysmography |
| OPLC | Optimum performance laminar chromatography (TLC/pressure) |
| OPLC | Over-pressure liquid chromatography |
| OPM | Office of Personnel Management |
| OPO | Optical Parametric Oscillator |
| OPPT | Office of Pollution Prevention & Toxics (US EPA) |
| OPS | Office of Pharmaceutical Sciences (US FDA) |
| OPV | Oral poliomyelitis vaccine |
| OR | Operations Research |
| OR | Operating Room(surgery) |
| ORA | Office of Regional Affairs (US FDA) |
| ORD | Optical Rotatory Dispersion |
| ORF | Open reading frame (molecular genetics) |
| Orn | Ornithine |
| ORNL | Oak Ridge National Laboratories |
| OS | Ocula sinistra |
| OSA | Optical Society of America |
| OSHA | (US) Occupational Safety and Health Administration |
| OSR | Oilseed rape(colza) |
| OSTP | Office of Science and Technology Policy |
| OSW | Office of Solid Waste (US EPA) |
| OTA | Office of Technology Assessment(US) |
| OTC | Over the counter (drug that does not require prescription) |
| OTC | Over The Counter (sales of pharmaceuticals, corporate stocks) |
| OTF | Optical Transfer Function |
| OVG | Oberverwaltungsgericht/ Higher Administrative Court |
| OVI | Organic Volatile Impurity |
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