| Oligodendroblastoma (M9460/3) | 
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  | Oligodendroglioma (M9450/3) | 
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  | Oligodendroma - see Oligodendroglioma | 
  | Oligodontia (see also Anodontia) 520.0 | 
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  | Oligophrenia (see also Retardation, mental) 319 | 
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  | Ollier's disease (chondrodysplasia) 756.4 | 
  | Omentitis (see also Peritonitis) 567.9 | 
  | Omentocele (see also Hernia, omental) 553.8 | 
  | Omentum, omental - see condition | 
  | Omphalitis (congenital) (newborn) 771.4 | 
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  | Omphalomesenteric duct, persistent 751.0 | 
  | Omphalorrhagia, newborn 772.3 | 
  | Omsk hemorrhagic fever 065.1 | 
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  | Oncocytoma (M8290/0) - see Neoplasm, by site, benign | 
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  | Oneirophrenia (see also Schizophrenia) 295.4 | 
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  | Onychia (with lymphangitis) 681.9 | 
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  | Onychitis (with lymphangitis) 681.9 | 
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  | Onychotrophia (see also Atrophy, nail) 703.8 | 
  | O'Nyong Nyong fever 066.3 | 
  | Onyxis (finger) (toe) 703.0 | 
  | Onyxitis (with lymphangitis) 681.9 | 
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  | Oocyte (egg) (ovum) | 
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  | Oophoritis (cystic) (infectional) (interstitial) (see also Salpingo-oophoritis) 614.2 | 
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  | Opacity, opacities | 
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