- Oculogyric
- Oculomotor syndrome 378.81
- Oddi's sphincter spasm 576.5
- Odelberg's disease (juvenile osteochondrosis) 732.1
- Odontalgia 525.9
- Odontoameloblastoma (M9311/0) 213.1
- Odontoclasia 521.05
- Odontoclasis 873.63
- Odontodysplasia, regional 520.4
- Odontogenesis imperfecta 520.5
- Odontoma (M9280/0) 213.1
- Odontomyelitis (closed) (open) 522.0
- Odontonecrosis 521.09
- Odontorrhagia 525.8
- Odontosarcoma, ameloblastic (M9290/3) 170.1
- Odynophagia 787.20
- Oesophagostomiasis 127.7
- Oesophagostomum infestation 127.7
- Oestriasis 134.0
- Ogilvie's syndrome (sympathicotonic colon obstruction) 560.89
- Oguchi's disease (retina) 368.61
- Ohara's disease (see also Tularemia) 021.9