- Ophiasis 704.01
- Ophthalmia (see also Conjunctivitis) 372.30
- Ophthalmitis - see Ophthalmia
- Ophthalmocele (congenital) 743.66
- Ophthalmoneuromyelitis 341.0
- Ophthalmopathy, infiltrative with thyrotoxicosis 242.0
- Ophthalmoplegia (see also Strabismus) 378.9
- Opisthognathism 524.00
- Opisthorchiasis (felineus) (tenuicollis) (viverrini) 121.0
- Opisthotonos, opisthotonus 781.0
- Opitz's disease (congestive splenomegaly) 289.51
- Opiumism (see also Dependence) 304.0
- Oppenheim's disease 358.8
- Oppenheim-Urbach disease or syndrome (necrobiosis lipoidica diabeticorum) 250.8 [709.3]
- Opsoclonia 379.59
- Optic nerve - see condition
- Orbit - see condition
- Orchioblastoma (M9071/3) 186.9
- Orchitis (nonspecific) (septic) 604.90
- Orf 051.2
- Organic - see also condition
- Oriental