• Obesity

See also: Weight Loss Surgery

• Obsessive-Compulsive Disorder

• Olivopontocerebellar Atrophy - See Multiple System Atrophy

• Open Angle (Chronic) Glaucoma - See Glaucoma

• Opsoclonus

• Optic Neuritis

• Oral Cancer - See Head and Neck Cancer

• Orbital Cellulitis

• Orchitis

• Osgood-Schlatter Disease

• Osteoarthritis

• Osteogenesis Imperfecta

• Osteogenic Sarcoma - See Bone tumors

• Osteomalacia

• Osteomyelitis

• Osteonecrosis

• Osteoporosis

• Osteosarcoma - See Bone tumors

• Otitis Media - See Ear Infection

• Otosclerosis

• Ovarian Cancer

• Ovarian Cysts