ICD-10-CM
- Obermeyer's relapsing fever (European)
- Obsessive-compulsive neurosis or reaction
- Occlusal wear, teeth
- Ochronosis (endogenous)
- Oculomotor syndrome
- Oddi's sphincter spasm
- Odontalgia
- Odontoclasia
- Odontodysplasia, regional
- Odontogenesis imperfecta
- Odontorrhagia
- Oguchi's disease
- OHS (obesity hypoventilation syndrome)
- Old (previous) myocardial infarction
- Oligocythemia
- Oligoencephalon
- Oligohidrosis
- Oligohydramnios
- Oligohydrosis
- Ollier's disease
- Omphalocele
- Omphalomesenteric duct, persistent
- Omphalorrhagia, newborn
- Omsk hemorrhagic fever
- Onanism (excessive)
- Oncovirus, as cause of disease classified elsewhere
- Oneirophrenia
- Onychocryptosis
- Onychogryphosis, onychogryposis
- Onycholysis
- Onychomadesis
- Onychomalacia
- Onychomycosis (finger) (toe)
- Onycho-osteodysplasia
- Onychophagia
- Onychophosis
- Onychoptosis
- Onychoschizia
- Onyxis (finger) (toe)
- Oophorocele
- Opalescent dentin (hereditary)
- Operational fatigue
- Ophiasis
- Ophthalmocele (congenital)
- Ophthalmoneuromyelitis
- Ophthalmopathy, Graves'
- Opisthognathism
- Opisthorchiasis (felineus) (viverrini)
- Opitz' disease
- Oppenheim's disease
- Opportunistic salpingectomy
- Orbitopathy (Graves') (thyroid)
- Orchioblastoma
- Orf (virus disease)
- Origin of both great vessels from right ventricle
- Ornithine metabolism disorder
- Ornithinemia (Type I) (Type II)
- Ornithosis
- Orthopnea
- Orthopoxvirus
- Osgood-Schlatter disease or osteochondrosis
- Osler's nodes
- Osler (-Weber)-Rendu disease
- Osmidrosis
- Osteochondrodystrophy
- Osteogenesis imperfecta
- Osteomyelofibrosis
- Osteomyelosclerosis
- Osteo-onycho-arthro-dysplasia
- Osteo-onychodysplasia, hereditary
- Osteopathia condensans disseminata
- Osteopetrosis (familial)
- Osteopoikilosis
- Osteopsathyrosis (idiopathica)
- Osteoradionecrosis, jaw (acute) (chronic) (lower) (suppurative) (upper)
- Osteosclerotic anemia
- Österreicher-Turner syndrome
- Ostrum-Furst syndrome
- Otalgia
- Otocephaly
- Otolith syndrome
- Otto's disease or pelvis
- Ovariocele
- Overbite (deep) (excessive) (horizontal) (vertical)
- Overconscientious personality
- Overexertion (effects) (exhaustion)
- Overfill, endodontic
- Overhanging of dental restorative material (unrepairable)
- Overjet (excessive horizontal)
- Overlap, excessive horizontal (teeth)
- Overprotection, child by parent
- Overuse, muscle NEC
- Overweight
- Overworked
- Ovotestis
- Owren's disease or syndrome (parahemophilia)
- Oxalosis
- Oxaluria
- Oxyuriasis
- Oxyuris vermicularis (infestation)
- Ozena