| OPSR | Office of Professional Standards Review |
|---|---|
| ORDS | Office of Research, Demonstration, and Statistics |
| OS | left eye [Lat. oculus sinister]; occipitosacral; occupational safety; office surgery; Omenn syndrome... |
| OSA | obstructive sleep apnea; Office of Services to the Aging; Optical Society of America; ovarian sectio... |
| OSRD | Office of Scientific Research and Development |