| Abbreviation | Term(s) |
| Obstetrics | OB |
| Obstructed (ing, ion) | OBST |
| Occupational history | OH |
| Occupational therapy | OT |
| Office visit | OV |
| Opening | OS |
| Operating room | OR |
| Operation | OP |
| Ophthalmology | OPHTH |
| Orthopedics | ORTH |
| Osteomyelitis | OSTEO |
| Otology | OTO |
| Ounce | OZ |
| Outpatient | OP |
| Outpatient clinic | OPD |
| Outpatient department | OPD |