- 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
- Omphalocele 756.79
- Omphalomesenteric duct, persistent 751.0
- Omphalorrhagia, newborn 772.3
- Omsk hemorrhagic fever 065.1
- Onanism 307.9
- Onchocerciasis 125.3
- Onchocercosis 125.3
- Oncocytoma (M8290/0) - see Neoplasm, by site, benign
- Ondine's curse 348.8
- Oneirophrenia (see also Schizophrenia) 295.4
- Onychauxis 703.8
- Onychia (with lymphangitis) 681.9
- Onychitis (with lymphangitis) 681.9
- Onycho-osteodysplasia 756.89
- Onychocryptosis 703.0
- Onychodystrophy 703.8
- Onychogryphosis 703.8
