Obesity
Obesity hypoventilation syndrome
Obsessive-compulsive disorder
Obstetric anal sphincter injury
Obstructive sleep apnea
Occupational liver disease
Onychomycosis
Open-angle glaucoma
Opioid use disorder
Opioids for pain
Oral candidiasis
Orolabial herpes
Oropharyngeal cancer
Orthodromic atrioventricular reentrant tachycardia
Orthostatic hypotension
Osteochondritis dissecans
Osteoporosis in men
Osteoradionecrosis of the jaw
Osteosarcoma
Ostomy surgery
Otitis media with effusion
Otosclerosis
Ovarian cancer
Ovarian cysts
Ovarian hyperstimulation syndrome
Overactive bladder