| O’nyong-nyong fever |
| OA |
| Obesity |
| Obesity hypoventilation syndrome (OHS) |
| OBS |
| Obsessive-compulsive disorder |
| Obsessive-compulsive neurosis |
| Obsessive-compulsive personality disorder |
| Obstetric sonogram |
| Obstetric ultrasonography |
| Obstructed airway |
| Obstruction of the ureteropelvic junction |
| Obstructive sleep apnea syndrome |
| Obstructive uropathy |
| Obstructive uropathy – bilateral – acute |
| Obstructive uropathy – bilateral – chronic |
| Obstructive uropathy – unilateral – acute |
| Obstructive uropathy – unilateral – chronic |
| Obtundation |
| Occipital-frontal circumference |
| Occlusion – renal artery |
| Occlusion – renal vein |
| Occupational asthma |
| Occupational bronchitis |
| Occupational hearing loss |
| OCD |
| OCG |
| Ocular albinism |
| Ocular larva migrans |
| Ocular melanoma |
| Ocular motility examination |
| Oculocutaneous albinism |
| Oculoglandular syndrome |
| Oculomotor palsy |
| Odynophagia |
| Off-pump coronary artery bypass |
| OG – infants |
| Ogilvie’s syndrome |
| Ohara disease |
| Ohio River Valley fever |
| OHSS |
| Oil |
| Oil of vitriol poisoning |
| Oil of wintergreen overdose |
| Oil-based paint poisoning |
| Oily hair |
| Oily skin |
| Oleander |
| Oligodendroglioma – adults |
| Oligodendroglioma – children |
| Oliguria |
| Olivopontocerebellar atrophy |
| Olivopontocerebellar degeneration |
| OME |
| Omphalocele |
| Omphalocele repair |
| Ondine’s curse |
| Onychocryptosis |
| Onycholysis |
| Onychomycosis |
| Oophoritis |
| OPCA |
| OPCAB |
| Open bite |
| Open browlift |
| Open cholecystectomy |
| Open heart surgery |
| Open injury bleeding |
| Open lung biopsy |
| Open pleural biopsy |
| Open prostatectomy |
| Open thyroid biopsy |
| Open wound |
| Open-angle glaucoma |
| Ophthalmalgia |
| Ophthalmia neonatorum |
| Ophthalmoscopy |
| Opiate withdrawal |
| Opioid intoxication |
| Opisthotonos |
| Oppositional defiant disorder |
| Optic glioma |
| Optic nerve atrophy |
| Optic nerve glioma |
| Optic neuritis |
| Oral cancer |
| Oral cholecystogram |
| Oral glucose tolerance test |
| Oral hypoglycemics overdose |
| Oral thrush |
| Oral ulcer |
| Orange crush overdose |
| Orbit CT scan |
