O
| O2 | Oxygen |
| OB | Obstetrics |
| OCPs | Oral contraceptive pills |
| OD | Overdose |
| O.D. | Doctor of Optometry |
| OD* | right eye |
| OM | Otitis media |
| OP | Outpatient; operative procedure |
| ORTH, Ortho | orthopedics |
| OS* | left eye; by mouth (pharmacology) |
| OU* | both eyes |
| OSA | Obstructive sleep apnea |
