| Abbreviation | Definition |
|---|---|
| o/a | on admission |
| OAD | Oral Antidiabetic Drug |
| OAR | Organ At Risk |
| OATS | Offering Alternative Therapy with Smiles |
| OATS | Osteochondral Autograft Transfer Surgery |
| OAUTH | Obafemi Awolowo University Teaching Hospital |
| OBB | Original Bidet Bottle |
| OBE | Out of Body Experience |
| OBG | Obstetrics and Gynecology |
| OBHK | Oceans Behavioral Hospital Kentwood |
| OC | Office of the Commissioner |
| OC | Oxycontin |
| OCCAM | Office of Cancer Complementary and Alternative Medicine |
| OCG | Oktibbeha County Hospital |
| OCH | Ozarks Community Hospital |
| OCMH | Osborne County Memorial Hospital |
| OCSG | Operation Capacity Study Group |
| OCT | Optical Coherence Tomography |
| OCTC | Ottawa Children's Treatment Centre |
| ODBT | Office of Devices, Blood, and Tissues |