Diagnosaurus
- Obesity
- Obsessive-compulsive disorder
- Obstruction of large intestine
- Obstruction of small intestine
- Obstructive hepatobiliary disease
- Obstructive sleep apnea
- Obstructive uropathy
- Occult GI bleeding
- Occupational pulmonary disease
- Ogilvie's syndrome
- Olfactory dysfunction
- Onchocerciasis (river blindness)
- Onycholysis
- Onychomycosis
- Open-angle glaucoma
- Opioid dependence and withdrawal
- Opioid overdose
- Optic neuritis
- Oral cancer
- Oral candidiasis
- Oral leukoplakia
- Oral lichen planus
- Orbital cellulitis
- Osteitis deformans
- Osteoarthritis
- Osteomalacia
- Osteomyelitis
- Osteoporosis
- Otitis externa
- Otitis media, acute
- Otitis media, chronic
- Otitis media, serous
- Ovarian cancer
- Ovarian failure, premature
- Ovarian tumor
- Overdose syndromes
- Overlap or mixed connective tissue disease