Diagnosaurus
- Obesity
- Obsessive-compulsive disorder
- Obstruction of large intestine
- Obstruction of small intestine
- Obstructive disease on PFTs
- Obstructive hepatobiliary disease
- Obstructive sleep apnea
- Obstructive uropathy
- Occult GI bleeding
- Occupational pulmonary disease
- Odynophagia
- Ogilvie's syndrome
- Olfactory dysfunction
- Oligomenorrhea & amenorrhea
- Onchocerciasis (river blindness)
- Onycholysis
- Onychomycosis
- Open-angle glaucoma
- Ophthalmoplegia
- Opioid dependence and withdrawal
- Opioid overdose
- Optic disc swelling
- Optic neuritis
- Oral cancer
- Oral candidiasis
- Oral lesions in HIV/AIDS
- Oral leukoplakia
- Oral lichen planus
- Oral ulcers
- Orbital cellulitis
- Orthopnea
- Osmolar gap
- Osteitis deformans
- Osteoarthritis
- Osteomalacia
- Osteomyelitis
- Osteoporosis
- Otalgia
- 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