Medical Dictionary: O

obesity: Excess body fat, usually defined as a body mass index of 30 or more.

obstructive sleep apnea: A disorder characterized by heavy snoring and interrupted breathing during sleep; often associated with obesity; a risk factor for stroke.

obstructive sleep apnea: Sleep apnea resulting from blockage of the airway.

omega-3 fatty acids: Beneficial fats also known as n-3 fatty acids. These are polyunsaturated fats in which the last double bond between carbon atoms is located three carbons from the end of the chain.

omega-6 fatty acids: Beneficial fats also known as n-6 fatty acids. These are polyunsaturated fats in which the last double bond between carbon atoms is located six carbons from the end of the chain.

onychomycosis: Medical term for toenail fungus.

open prostatectomy: A surgical procedure in which prostate tissue is removed through an incision in the abdomen.

ophthalmoscope: An instrument with a light and mirrors for examining the deep interior of the eye.

opioid: Any substance, either natural or synthetic, that behaves in the body like an opium-derived drug. Opioids are potent painkillers.

optic nerve: A “cable” that emanates from the back of the eye, consisting of specialized nerve fibers that transmit visual impulses to the brain.

oral glucose tolerance test: A two-part test that first measures blood glucose levels after an overnight fast, and then two hours after consumption of a sugary solution; sometimes used in conjunction with fasting plasma glucose test.

orbit: The bony socket that surrounds the eyeball.

orchiectomy: A surgical procedure to slow the growth of prostate cancer that involves the removal of the testosterone-producing testicles.

orthosis: A custom shoe insert, usually constructed of foam or a composite material; helps cushion or realign the foot.

orthostatic hypotension: A sudden drop in blood pressure when someone stands or gets out of bed, causing dizziness.

ossicles: The malleus, incus, and stapes — the three bones in the middle ear that move in response to sound vibrations.

osteoarthritis: A joint disease common with aging, characterized by progressive deterioration of the cartilage that lines the joints; also called degenerative joint disease.

osteoblasts: Bone-forming cells.

osteoclasts: Bone-removing cells.

osteocytes: Osteoblasts that have completed their bone-forming function and have become trapped in new bone tissue, evolving into structural bone cells.

osteons: The basic units of compact bone, consisting of tightly packed concentric rings of tissue with a blood vessel running through the central channel.

osteopenia: Low bone density, but not enough thinning to be classified as osteoporosis.

osteophyte: An outgrowth of bone on the margins of a joint or intervertebral disk, commonly called a bone spur, generally a result of osteoarthritis.

osteoporosis: A degenerative disease that decreases the density of bone, leaving it vulnerable to fracture.

otic capsule: The bone that surrounds the labyrinth of the inner ear.

otitis externa: A bacterial or fungal infection of the skin lining the ear canal of the outer ear. It’s most common during the summer and in hot, humid climates. Also called swimmer’s ear.

otitis media: An infection of the middle ear, which can be acute or chronic.

otosclerosis: Abnormal bony growth that keeps one of the three tiny bones in the middle ear from moving, thereby preventing sound waves from passing through the ear.

outer ear: The external part of the ear, or auricle, as well as the ear canal and the eardrum.

overactive bladder: Frequent urination and urges to urinate; a popular name for detrusor instability.

overt proteinuria: A condition that may develop after microalbuminuria and shows worsening kidney function; precedes chronic kidney disease.

overweight: A condition marked by a body mass index of 25 to 30.

oxidation: A process in which oxygen combines with a substance, altering its structure and changing or destroying its normal function.