| O3DCLCC | Oriented 3 Dimensionally Crosslinked Liquid Crystal Chiral |
| OA | Osteo Arthritis |
| OAB | Over Active Bladder |
| OAC | Oral Anti Coagulation |
| OAG | Open-Angle Glaucoma |
| OAP | Overrunning alternator pulleys |
| OAR | Organs At Risk |
| OAS | Orbital Atherectomy System |
| OAV | Ovine Adeno Virus |
| OBD | Opioid-induced Bowel Dysfunction |
| OBS | Organic Based Stabilizer (PVC) |
| OBSH | 4,4'-Oxy bis Benzene Sulfonyl Hydrazide |
| OBT | Optimized Background Therapy |
| OCA | O Carboxy Anhydride |
| OCBAL | Ortho Chloro Benz AL dehyde |
| OCD | Obsessive Compulsive Disorders |
| OCEB | Ortho Chloro Ethyl Benzene |
| OCL | Osteo Clast |
| OCRP | Oral Controlled Release Patch |
| OCS | Organosilanized Colloidal Silica |
| OCT | Olefins Conversion Technology (Methathesis) |
| OCU | Olefin Conversion Unit |
| OD | 1,2-Octane Diol |
| OD | Octa Decadienoic (acid) |
| OD | Once Daily |
| ODAC | Oncologic Drugs Advisory Committee (FDA) |
| ODC | Ornithine De Carboxylase |
| ODC | Oxygen Depolarised Cathode |
| ODC | Oxygen Depleting Cathode |
| ODCB | Ortho Di Chloro Benzene |
| ODD | Oppositional Defiant Disorder |
| ODD | Orally Disintegrating Dosage (Pharma.>Drug Delivery) |
| ODD | Orphan Drug Designation |
| ODP | Ozone Depletion Potential |
| ODP Eye | Optic Disc Pit maculopathy |
| ODPA | 4,4'-Oxy Di (Phthalic Anhydride) |
| ODPEG | Octa Decadienoic (acid) Poly Ethyleno Glycol (ester) |
| ODPEGSu | ODPEG Succinic acid deriv. |
| ODT | Orally Disintegrating Tablets |
| OEB | Occupational Exposure Band |
| OED | Oleoyl Estrone Developments |
| OEL | Organic Electro Luminescence |
| OEL | Occupational Exposure Limit |
| OEM | Original Equipment Manufacturing |
| OFS | Organic Formulated Stabilizer (PVC) |
| OGG | Oxo Guanine Glycosylase |
| OGTT | oral glucose tolerance test |
| OHC | Over Head Camshafts |
| OHP | Organo Hydroxylase Phosphorus (Enzyme) |
| OHSV | Oncolytic Herpes Simplex Virus |
| OHT | Ocular HyperTension |
| OHV | Over Head Valves |
| OIA | Optical Imaging Agent |
| OIC | Opioid Induced Constipation |
| OIP | Osteoclast Inhibitor Protein |
| OIT | N-n-Octyl Iso Thiazolin-3-one (Biocides) |
| OL | On Line |
| OLED | Organic Light Emitting Diode |
| OLTP | On Line Transaction Processing |
| OM | Oral Mucositis |
| OMC | Octyl Methoxy Cinnamate |
| OMD | Oral Mucoadhesive Dosage |
| OMG | Optical Magnetic Gradiometer |
| OMP | Orotidine Mono Phosphate |
| OMRI | Overhauser Magnetic Resonance Imaging |
| OMT | optimal medical therapy |
| ONC | Octa Nitro Cubane (Explosive) |
| ONR | Office of Naval Research (USA) |
| OP | Organo Phosphate |
| OPAA | Organo Phosphorous Acid Anhydrolase |
| OPC | Oro Pharyngeal Candidiasis |
| OPCW | Organisation for the Prohibition of Chemical Weapons |
| OPEP | oscillatory positive expiratory pressure |
| OPG | Osteo Prote Gerin (Protein) |
| OPH | Organo Phosphorus Hydrolase |
| OPLC | Over Pressured Layer Chromatography |
| OPO | Optical Parametric Oscillator (Photonics) |
| OPP | Ortho Phenyl Phenol (biocide) |
| OPP | Oriented Poly Propylene (packaging film) |
| oPPV | Oligo-P-PhenyleneVinylene |
| OPRD | Organic Process Research & Development (ACS journal) |
| OPV | Organic Photo Voltaics |
| OR | Oil Repellency (textiles) |
| OR | Operating Room (Surgery) |
| ORAC | Oxygen Radical Absorption Capacity |
| ORC | Oxidized Regenerated Cellulose |
| ORF | Open Reading Frame (Coding part of DNA seq) |
| ORG | ORGanic, ORGanization |
| ORL | Opioid Receptor Like |
| ORR | overall response rate |
| ORS | ORal Suspension |
| OS | overall survival |
| OSA | Obstructive Sleep Apnea |
| OSAB | Oxidative Stressed Autologous Blood |
| OSB | Oriented Strand Board |
| OSBAL | Ortho Sulfonyl Benz AL dehyde |
| OSCS | Over Sulfated Chondroitin Sulfate |
| OSDA | Organic Structure Directing Agent |
| OSDF | Oral Solid Dosage Formulation |
| OSHA | Occupational Safety & Health Administration (USA) |
| OSN | Organic Solvent Nanofiltration |
| OSRM | Oxidative Steam Reforming of Methanol |
| OTC | Over The Counter (Pharma.) |
| OTEC | Ocean Thermal Energy Conversion |
| OTL | Oxidized Transport Layer (semiconducting polymer system) |
| OTO | Occupant To Occupant |
| OTP | Ortho Tolyl Pentene |
| OTP | One Time Programmable |
| OTPA | Otonecine Type Pyrrolizidine Alkaloids |
| OTS | n-Octyl Trichloro Silane |
| OUD | opioid use disorder |
| OVAT | One Variable At a Time (Pharma. Process.) |
| OVD | Ophthalmic ViscoSurgical Device |
| OVHS | OVarian Histo Scanning |