| Abbreviation | Term(s) |
| OB | Obstetrics |
| OBST | Obstructed (ing, ion) |
| OD | Right eye (oculus dexter) |
| OH | Occupational history |
| OP | Operation |
| OP | Outpatient |
| OPD | Outpatient clinic; department |
| OPHTH | Ophthalmology |
| OR | Operating room |
| ORTH | Orthopedics |
| OS | Bone |
| OS | Left eye (oculus sinister) |
| OS | Mouth |
| OS | Opening |
| OSTEO | Osteomyelitis |
| OT | Occupational therapy |
| OTO | Otology |
| OU | Each eye (oculus uterque) |
| OV | Office visit |
| OZ | Ounce |