- Obesity
- Obesity-hypoventilation syndrome
- Obsessive-compulsive behavior, in Tourette's syndrome
- Obsessive-compulsive disorder
- Obstruction
- Obstructive cardiomyopathy, hypertrophic
- Obstructive sleep apnea
- Obstructive voiding symptoms
- Occlusion
- Occult gastrointestinal bleeding
- Occupational asthma
- Occupational pulmonary disease
- Ochronosis
- Octreotide acetate
- Ocular muscular dystrophy
- Ocular toxocariasis
- Oculopharyngeal muscular dystrophy
- Ocupress
- Odynophagia
- Ofloxacin 0.3% solution, as ophthalmic agent
- Ogilvie's syndrome
- Olanzapine
- Olfactory dysfunction
- Oligodendroglioma
- Oligomenorrhea
- Olopatadine hydrochloride 0.1% solution
- Onchocerca volvulus
- Onchocerciasis
- Ondansetron
- ONTAK
- Onycholysis
- Onychomycosis
- Ophthalmic agents, topical
- Ophthalmic-related headache
- Ophthalmocysticercosis
- Opioid agonist analgesics
- Opioid dependency
- Opioid poisoning
- Opioid withdrawal
- Opisthorchiasis
- Opisthorchis felineus
- Opisthorchis viverrini
- Opportunistic infections and malignancies
- Optic disc swelling
- Optic neuritis
- OptiPranolol
- Oral cancer
- Oral candidiasis
- Oral contraceptives
- Oral glucose tolerance test, Diabetes Expert Committee evaluation criteria for
- Oral lesions in HIV/AIDS
- Oral lichen planus
- Oral ulcers
- Oramorph
- Orbital cellulitis
- Organophosphate poisoning
- Organosphosphate poisoning
- Orgaran
- Orienta tsutsugamushi
- Orinase
- Oropharyngeal tumors
- Orthomyxovirus
- Orthopnea, or nocturnal dyspnea
- Orthostatic syncope
- Orudis
- Oruvail
- Osmolar gap
- Osmotic diarrhea
- Osteitis deformans
- Osteoarthritis
- Osteoarthritis, diagnostic value of joint pattern
- Osteogenic sarcoma, treatment of
- Osteomalacia
- Osteomyelitis
- Osteoporosis
- Otitis, Haemophilus, drugs for
- Otitis externa
- Otitis media, acute
- Otitis media, chronic
- Otitis media, serous
- Ototoxicity
- Ovarian cancer
- Ovarian tumor
- Overdose
- Overflow incontinence
- Overuse syndromes, knee
- Overweight
- Ovulation, induction of
- Ovulation bleeding
- Oxaliplatin
- Oxaprozin
- Oxazepam
- Oxcarbazepine
- Oxiconazole
- Oxistat
- Oxycodone
- Oxycodone Controlled Release
- Oxycontin
- Oxygen therapy, home, Medicare requirements for
- Oxymorphone