| ORD | optical rotatory dispersion; oral radiation death |
|---|---|
| OROS | oral osmotic |
| OS | left eye [Lat. oculus sinister]; occipitosacral; occupational safety; office surgery; Omenn syndrome... |
| OSCC | oral squamous cell carcinoma |
| OTD | oculotrichodysplasia; oral temperature device; organ tolerance dose |