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obesity: A body weight that is much higher than is healthy. Defined as having a body mass index of 30 or more. Obesity puts a person at greater risk of developing numerous chronic diseases.

object of addiction: The psychoactive drug or rewarding behavior with which a person with addiction has a pathological relationship.

obstructive sleep apnea: A disorder marked by heavy snoring and interrupted breathing during sleep. It increases the risk of developing high blood pressure and having a stroke and is more common in people who are obese.

occipital lobe: The region in the back of the brain responsible for visual processing.

occlusion: The closing or blocking of a hollow organ or body part.

occult: Something not visible to the naked eye but seen under a microscope or through lab tests.

omega-3 fatty acids: Beneficial fats, also known as n-3 fatty acids. These are polyunsaturated fat found in fatty fish, such as salmon.

omega-6 fatty acids: Fatty acids found in certain foods that the body needs for good health but can't make on its own. Also known as n-6 fatty acids.

oncogene: A gene that, under certain conditions, can cause cancer.

oncologist: A physician who deals with the diagnosis and treatment of cancer. There are three types—medical oncologists, radiation oncologists, and surgical oncologists.

Ondine's curse: A rare and potentially fatal disorder in which a person may stop breathing, especially at night. Also called congenital central hypoventilation syndrome.

onychomycosis: Toenail fungus.

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

ophthalmologist: A physician who specializes in treating the eye and eye disease.

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

opiate: Any painkilling drug such as morphine or codeine derived from the opium poppy.

opioid: Any narcotic, natural or synthetic, that behaves in the body like an opium-derived drug.

optic disk: The front surface of the optic nerve, where all the retinal nerve fibers come together to carry an image to the brain.

optic nerve: A cable of specialized nerve fibers that transmit visual impulses from the eye to the brain.

optician: A technician who helps select and fit eyeglasses or contact lenses for people with vision problems.

optimism: A characteristic frame of mind that leads a person to expect positive outcomes and to view the world as a positive place.

optometrist: A health care professional licensed to examine the eye, and diagnose and treat some eye diseases.

oral glucose tolerance test: A test to check for diabetes. It involves fasting overnight and having blood sugar levels checked before and after drinking a sugary solution.

oral mucosa: The layer of soft pinkish tissue that lines the interior of the mouth.

orbit: The bony socket that holds the eyeball.

orbital irradiation: X-ray treatment to the eye; sometimes used in more serious cases of Graves' eye disease.

orchiectomy: Surgery to remove the testicles. Usually done to remove a cancerous testicle or to lower testosterone levels and slow or halt the growth of prostate cancer.

organic matrix: The protein framework of bone tissue.

organonitrile: A chemical found in cruciferous vegetables that may have anti-cancer properties.

orgasm: The series of pleasurable, rhythmic muscle contractions that mark the peak of sexual arousal and the release of muscle tension.

orthopedist: A medical doctor who specializes in correcting disorders of the bones, joints, muscles, and tendons.

orthosis: A custom shoe insert that helps cushion or realign the foot.

orthostatic hypotension: A sudden drop in blood pressure upon standing or getting out of bed, causing dizziness.

osseointegration: A process in which bone heals around an implant to create a stable anchor.

ossicles: Three bones in the middle ear that move in response to sound vibrations.

ossification: The process by which bone is formed.

osteoarthritis: A joint disease in which the cartilage that lines the joints slowly deteriorates. Also called degenerative joint disease.

osteoblasts: Cells that build bone tissue.

osteoclasts: Cells that remove bone tissue.

osteocytes: A cell that is embedded in fully formed bone.

osteomalacia: A condition in which bones are soft and weak, usually due to a lack of vitamin D or an inability of the body to use vitamin D properly.

osteomyelitis: A bone infection caused by bacteria or fungi.

osteons: The building blocks of compact bone, the hard, tightly-packed tissue that forms the outer shell of bones.

osteopath: A doctor licensed to practice medicine, perform surgery, and prescribe drugs. The training is similar to that of a regular M.D., but more emphasis is placed on the importance of the musculoskeletal system and the body's ability to heal itself.

osteopenia: Mild thinning and weakening of the bones; bone density is lower than normal but not low enough to be classified as osteoporosis.

osteophyte: An outgrowth of bone on a joint or spinal disk; commonly called a bone spur.

osteoporosis: Significant thinning and weakening of bones over time, making them vulnerable to breaks.

osteotomy: An operation in which bone is cut to change its alignment or shorten or lengthen it.

otic capsule: The bony shell that surrounds the inner ear.

otitis externa: An infection of the skin lining the ear canal of the outer ear. Also called swimmer's ear.

otitis media: An infection of the middle ear.

otosclerosis: An abnormal bony growth in the middle ear that causes hearing loss.

outbreak: Synonymous with epidemic. Sometimes used to refer to a local epidemic as compared to a larger, general epidemic.

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

outpatient: A person who receives treatment at a hospital or other medical facility but does not stay overnight.

output: The loudest sound that a hearing aid can produce.

ovariectomy: Surgical removal of one or both ovaries.

overactive bladder: Frequent urination and urges to urinate.

overt proteinuria: A condition of declining kidney function. It is part of the progression of kidney disease, developing after microalbuminuria (when a damaged kidney begins to leak small amounts of a protein called albumin into urine) and before chronic kidney disease.

overweight: A body weight above the healthy range but not obese. Usually defined as a body mass index of 25 to 29.9.

ovulation: The release of a mature egg from the ovary, at which time it is available to be fertilized by sperm.

oxidant: An unstable molecule in the body that plays a role in aging and can damage tissue. Also known as a free radical.

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

oxygenated blood: Blood that has moved through the lungs where it has absorbed oxygen; oxygenated blood moves from the lungs into the heart, from which it is then pumped throughout the body.