Otitis media
Otitis externa
Otosclerosis
Omphalocele
Optic nerve hypoplasia
Osteogenesis imperfecta
Opsoclonus myoclonus syndrome
Oral candidiasis
Orchitis
Ototoxicity
Obsessive-compulsive disorder
Osteoporosis
Optic neuritis
Obsessive-compulsive personality disorder
Osteosarcoma
Onchocerciasis
Obstetric fistula
Oxycephaly
Orthostatic hypotension
Oculocerebrorenal syndrome
Orbital cellulitis
Oculopharyngeal muscular dystrophy
Odontoma
Optic nerve sheath meningioma
Osteopenia
Optic atrophy
Oral cancer
Ovarian cancer
Osteoarthritis
Osteomalacia
Osteomyelitis