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
- Odynophagia
- Olfactory dysfunction
- Oligomenorrhea & amenorrhea
- Onycholysis
- Ophthalmoplegia
- Optic disc swelling
- Oral lesions in HIV/AIDS
- Oral leukoplakia
- Oral ulcers
- Orthopnea
- Osmolar gap
- Otalgia
- Ovarian cancer
- Ovarian tumor
- Overdose syndromes