| Opacity, opacities | 
  |  | 
  | Opalescent dentin (hereditary) 520.5 | 
  | Open, opening | 
  |  | 
  | Operation | 
  |  | 
  |  | 
  | Operative - see condition | 
  |  | 
  |  | 
  |  | 
  |  | 
  |  | 
  | 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 | 
  |  | 
  |  | 
  | 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] | 
  |  | 
  |  | 
  | Optic nerve - see condition | 
  | Orbit - see condition | 
  | Orchioblastoma (M9071/3) 186.9 | 
  | Orchitis (nonspecific) (septic) 604.90 | 
  |  | 
  |  | 
  | Organic - see also condition | 
  |  | 
  | Oriental | 
  |  | 
  | Orientation | 
  |  | 
  | Orifice - see condition | 
  | Origin, both great vessels from right ventricle 745.11 | 
  | Ormond's disease or syndrome 593.4 | 
  |  | 
  |  | 
  | Orodigitofacial dysostosis 759.89 | 
  |  | 
  | Orotaciduria, oroticaciduria (congenital) (hereditary) (pyrimidine deficiency) 281.4 | 
  |  | 
  |  | 
  |  | 
  |  | 
  | 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 | 
  |  | 
  | Osseous - see condition | 
  | Ossification | 
  |  | 
|  |  |