- Orientation
- Orifice - see condition
- Origin, both great vessels from right ventricle 745.11
- Ormond's disease or syndrome 593.4
- Ornithosis 073.9
- Orodigitofacial dysostosis 759.89
- Oropouche fever 066.3
- Orotaciduria, oroticaciduria (congenital) (hereditary) (pyrimidine deficiency) 281.4
- Oroya fever 088.0
- Orthodontics V58.5
- Orthopnea 786.02
- Os, uterus - see condition
- Osgood-Schlatter
- Osler's
- Osler-Rendu disease (familial hemorrhagic telangiectasia) 448.0
- Osler-Vaquez disease (M9950/1) (polycythemia vera) 238.4
- Osler-Weber-Rendu syndrome (familial hemorrhagic telangiectasia) 448.0
- Osmidrosis 705.89
- Osseous - see condition
- Ossification
- Osteitis (see also Osteomyelitis) 730.2
- Osteo-onycho-arthro dysplasia 756.89