- Österreicher-Turner syndrome, Q87.2
- Obermeyer\’s relapsing fever (European), A68.0
- Obesity, E66.9
- Oblique
- Obliteration
- Observation (following) (for) (without need for further medical care), Z04.9
- Obsession, obsessional state, F42.8
- Obsessive-compulsive neurosis or reaction, F42.8
- Obstetric embolism, septic
- Obstetrical trauma (complicating delivery), O71.9
- Obstipation
- Obstruction, obstructed, obstructive
- Obturator
- Occlusal wear, teeth, K03.0
- Occlusio pupillae
- Occlusion, occluded
- Occult
- Occupational
- Ochlophobia
- Ochronosis (endogenous), E70.29
- Ocular muscle
- Oculogyric crisis or disturbance, H51.8
- Oculomotor syndrome, H51.9
- Oculopathy
- Oddi\’s sphincter spasm, K83.4
- Odontalgia, K08.89
- Odontoameloblastoma
- Odontoclasia, K03.89
- Odontodysplasia, regional, K00.4
- Odontogenesis imperfecta, K00.5
- Odontoma (ameloblastic) (complex) (compound) (fibroameloblastic)
- Odontomyelitis (closed) (open), K04.01
- Odontorrhagia, K08.89
- Odontosarcoma, ameloblastic, C41.1
- Oestriasis
- Oguchi\’s disease, H53.63
- Ohara\’s disease
- OHS (obesity hypoventilation syndrome), E66.2
- Oidiomycosis
- Oidium albicans infection
- Old age (without mention of debility), R54
- Old myocardial infarction (previous), I25.2
- Olfactory
- Oligemia
- Oligoastrocytoma
- Oligocythemia, D64.9
- Oligodendroblastoma
- Oligodendroglioma
- Oligodontia
- Oligoencephalon, Q02
- Oligohidrosis, L74.4
- Oligohydramnios, O41.0-
- Oligohydrosis, L74.4
- Oligomenorrhea, N91.5
- Oligophrenia
- Oligospermia, N46.11
- Oligotrichia
- Oliguria, R34
- Ollier\’s disease, Q78.4
- Omenotocele
- Omentitis
- Omentum, omental
- Omphalitis (congenital) (newborn), P38.9
- Omphalocele, Q79.2
- Omphalomesenteric duct, persistent, Q43.0
- Omphalorrhagia, newborn, P51.9
- Omsk hemorrhagic fever, A98.1
- Onanism (excessive), F98.8
- Onchocerciasis, onchocercosis, B73.1
- Oncocytoma
- Oncovirus, as cause of disease classified elsewhere, B97.32
- Ondine\’s curse
- Oneirophrenia, F23
- Onychauxis, L60.2
- Onychia
- Onychitis
- Onycho-osteodysplasia, Q87.2
- Onychocryptosis, L60.0
- Onychodystrophy, L60.3
- Onychogryphosis, onychogryposis, L60.2
- Onycholysis, L60.1
- Onychomadesis, L60.8
- Onychomalacia, L60.3
- Onychomycosis (finger) (toe), B35.1
- Onychophagia, F98.8
- Onychophosis, L60.8
- Onychoptosis, L60.8
- Onychorrhexis, L60.3
- Onychoschizia, L60.3
- Onyxis (finger) (toe), L60.0
- Onyxitis
- Oophoritis (cystic) (infectional) (interstitial), N70.92
- Oophorocele, N83.4-
- Opacity, opacities
- Opalescent dentin (hereditary), K00.5
- Open, opening
- Operational fatigue, F48.8
- Operative
- Operculitis
- Operculum
- Ophiasis, L63.2
- Ophthalmia, H10.9
- Ophthalmitis
- Ophthalmocele (congenital), Q15.8
- Ophthalmoneuromyelitis, G36.0
- Ophthalmoplegia
- Opioid (s)
- Opisthognathism, M26.09
- Opisthorchiasis (felineus) (viverrini), B66.0
- Opitz\’ disease, D73.2
- Opiumism
- Oppenheim-Urbach disease (necrobiosis lipoidica diabeticorum)
- Oppenheim\’s disease, G70.2
- Optic nerve
- Orbit
- Orchioblastoma, C62.9-
- Orchitis (gangrenous) (nonspecific) (septic) (suppurative), N45.2
- Orf (virus disease), B08.02
- Organic
- Orgasm
- Oriental
- Orifice
- Origin of both great vessels from right ventricle, Q20.1
- Ormond\’s disease (with ureteral obstruction), N13.5
- Ornithine metabolism disorder, E72.4
- Ornithinemia (Type I) (Type II), E72.4
- Ornithosis, A70
- Orotaciduria, oroticaciduria (congenital) (hereditary) (pyrimidine deficiency), E79.8
- Orthodontics
- Orthopnea, R06.01
- Orthopoxvirus, B08.09
- Os, uterus
- Osgood-Schlatter disease or osteochondrosis, M92.52-
- Osler-Rendu disease (-Weber), I78.0
- Osler\’s nodes, I33.0
- Osmidrosis, L75.0
- Osseous
- Ossification
- Osteitis
- Osteo-onycho-arthro-dysplasia, Q87.2
- Osteo-onychodysplasia, hereditary, Q87.2
- Osteoarthritis, M19.90
- Osteoarthropathy (hypertrophic), M19.90
- Osteoarthrosis (degenerative) (hypertrophic) (joint)
- Osteoblastoma
- Osteochondritis
- Osteochondroarthrosis deformans endemica
- Osteochondrodysplasia, Q78.9
- Osteochondrodystrophy, E78.9
- Osteochondrolysis
- Osteochondroma
- Osteochondromatosis, D16.9
- Osteochondromyxosarcoma
- Osteochondropathy, M93.90
- Osteochondrosarcoma
- Osteochondrosis
- Osteoclastoma, D48.0
- Osteodynia
- Osteodystrophy, Q78.9
- Osteofibroma
- Osteofibrosarcoma
- Osteogenesis imperfecta, Q78.0
- Osteogenic
- Osteolysis, M89.50
- Osteoma
- Osteomalacia, M83.9
- Osteomyelitis (general) (infective) (localized) (neonatal) (purulent) (septic) (staphylococcal) (streptococcal) (suppurative) (with periostitis), M86.9
- Osteomyelofibrosis, D47.4
- Osteomyelosclerosis, D75.89
- Osteonecrosis, M87.9
- Osteopathia condensans disseminata, Q78.8
- Osteopathy
- Osteopenia, M85.8-
- Osteoperiostitis
- Osteopetrosis (familial), Q78.2
- Osteophyte, M25.70
- Osteopoikilosis, Q78.8
- Osteoporosis (female) (male), M81.0
- Osteopsathyrosis (idiopathica), Q78.0
- Osteoradionecrosis, jaw (acute) (chronic) (lower) (suppurative) (upper), M27.2
- Osteosarcoma (any form)
- Osteosclerosis, Q78.2
- Osteosclerotic anemia, D64.89
- Osteosis
- Ostium
- Ostrum-Furst syndrome, Q75.8
- Otalgia
- Otitis (acute), H66.90
- Otocephaly, Q18.2
- Otolith syndrome
- Otomycosis NEC (diffuse), B36.9
- Otoporosis
- Otorrhagia (nontraumatic), H92.2-
- Otorrhea, H92.1-
- Otosclerosis (general), H80.9-
- Otospongiosis
- Otto\’s disease or pelvis, M24.7
- Outcome of delivery, Z37.9
- Outlet
- Ovalocytosis (congenital) (hereditary)
- Ovarian
- Ovariocele, N83.4-
- Ovaritis (cystic)
- Ovary, ovarian
- Overactive
- Overactivity, R46.3
- Overbite (deep) (excessive) (horizontal) (vertical), M26.29
- Overbreathing
- Overconscientious personality, F60.5
- Overdevelopment
- Overdistension
- Overdose, overdosage (drug)
- Overeating, R63.2
- Overexertion (effects) (exhaustion), T73.3
- Overexposure (effects), T73.9
- Overfeeding
- Overfill, endodontic, M27.52
- Overgrowth, bone
- Overhanging of dental restorative material (unrepairable), K08.52
- Overheated (places) (effects)
- Overjet (excessive horizontal), M26.23
- Overlaid, overlying (suffocation)
- Overlap, excessive horizontal (teeth), M26.23
- Overlapping toe (acquired)
- Overload
- Overnutrition
- Overproduction
- Overprotection, child by parent, Z62.1
- Overriding
- Overstrained, R53.83
- Overuse, muscle NEC, M70.8-
- Overweight, E66.3
- Overworked, R53.83
- Oviduct
- Ovotestis, Q56.0
- Ovulation (cycle)
- Ovum
- Owren\’s disease or syndrome (parahemophilia), D68.2
- Ox heart
- Oxalosis, E72.53
- Oxaluria, E72.53
- Oxycephaly, oxycephalic, Q75.0
- Oxyuriasis, B80
- Oxyuris vermicularis (infestation), B80
- Ozena, J31.0