List of medical abbreviations: O
# List of medical abbreviations: O

Sortable table :

| Abbreviation | Meaning |
| - | - |
| o | Nothing [ō—letter o with overbar] |
| $O_2$ | oxygen |
| OA | osteoarthritis |
| OAB | Overactive bladder |
| OAF | osteoclast activating factor |
| OB | Occult blood |
| OB OB-GYN ob-gyne | obstetrics and gynecology |
| Obl | oblique |
| OBS | organic brain syndrome |
| Occ | occasional |
| OCD | obsessive-compulsive disorder |
| OCG | oral cholecystogram |
| OCNA | Orthopedic Clinics of North America |
| OCP | oral contraceptive pill |
| OCT | optical coherence tomography |
| OD | once daily (from Latin omne in die) right eye (from Latin oculus dexter) overdose occupational disease |
| od | every day, usually regarded as once daily (from Latin omni die). Sometimes written in lowercase. |
| ODC | ornithine decarboxylase |
| OE | otitis externa |
| O/E | on examination |
| OFC | orbitofrontal cortex |
| OGTT | oral glucose tolerance test |
| OHL | oral hairy leukoplakia |
| OHS | Obesity hypoventilation syndrome |
| OHT | Orthotopic heart transplantation |
| Oint | ointment |
| OM | otitis media |
| om | every morning (from Latin omni mane). Generally written in lowercase. |
| OME | otitis media with effusion (fluid in the inner ear without other symptoms) |
| OMS | Opsoclonus-myoclonus syndrome |
| on | every night (from Latin omni nocte). Generally written in lowercase. |
| O/N | overnight |
| OOB | out of bed |
| OP | outpatient department Osteoporosis |
| O&P | ova and parasites |
| OPAT | Outpatient parenteral antibiotic therapy |
| OPD | outpatient department |
| OPPT | oriented to person, place, and time |
| OPV | outpatient visit |
| OR | operating room (aka operating theatre) odds ratio |
| ORIF | open reduction internal fixation |
| ORSA | oxacillin-resistant staphylococcus aureus |
| ORT | oral rehydration therapy |
| OS | left eye (from Latin oculus sinister) orthopedic surgery overall survival |
| OSA | obstructive sleep apnea |
| OSH | outside hospital |
| Osm | osmolarity |
| Osteo | osteomyelitis |
| OT | occupational therapy |
| OTC | over-the-counter drug |
| OTD | out the door (discharged) |
| OTPP | oriented to time, place, and person |
| OTTR | Organ Transplant Tracking Record |
| OU | both eyes (from Latin oculi uterque) |
| OV | office visit (see ambulatory care) |
| oz | ounce |
