Obesity
Obsessive compulsive disorder (OCD)
Obstructive sleep apnea
Occupational asthma
Ocular rosacea
Oculoglandular tularemia
Onychomycosis
Oppositional defiant disorder (ODD)
Optic neuritis
Oral lichen planus
Oral thrush
Orchitis
Oropharyngeal tularemia
Orthostatic hypotension (postural hypotension)
Osgood Schlatter disease
Osteoarthritis
Osteochondritis dissecans
Osteomalacia
Osteomyelitis
Osteoporosis
Ovarian cancer
Ovarian cysts
Ovarian hyperstimulation syndrome
Overactive bladder