o.c. - Oculentum 
o.d. - Oculus Dexter 
o.d. - Omni Die 
o.l. - Oculus Laevus 
o.m. - Omni Mane 
o.n. - Omni Nocte 
o.r. - Oculus Rectus 
o.s. - Oculus Sinister 
o.u. - Oculus Uterque 
o.u. - Oculus Utro 
OAT - Oligo-Astheno-Teratozoospermie 
OCD - Obsessive-Compulsive Disorder 
OCR - Office for Civil Rights 
OD - Office on Disability 
od. - Omni Die 
OEC - olfactory ensheathing cell 
OGC - Office of the General Counsel 
OGHA - Office of Global Health Affairs 
OIG - Office of Inspector General 
OMB - Office of Management and Budget 
OMH - Office of Minority Health 
OMHA - Office of Medicare Hearings and… 
OMSOC - Occupational Medicine Speciali… 
ONC - Office of the National Coordinat… 
OPHS - Office of Public Health and Sci… 
ORR - Office of Refugee Resettlement 
ORS - outer root sheath 
OS - Office of the Secretary 
OSG - Office of the Surgeon General