| Meaning | ||
| o | Nothing [ō—letter o with overbar] | |
| O2 | 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)[1] right eye (from Latin oculus dexter) overdose occupational disease | |
| 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 | ||
| 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 |