Obesity hypoventilation syndrome (OHS) Obsessive-compulsive disorder Obsessive-compulsive personality disorder Obstruction of the ureteropelvic junction Obstructive uropathy OHSS Olivopontocerebellar atrophy Omphalocele Ondines curse Oophoritis Oppositional defiant disorder Optic nerve atrophy Optic neuritis Orbital cellulitis Osgood-Schlatter disease Osmotic diuresis Osteitis deformans Osteitis fibrosa Osteogenic sarcoma Osteomalacia Osteomalacia in children Osteomyelitis Osteoporosis Otosclerosis Ovarian hypofunction Ovarian overproduction of androgens Ozena