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Glossaries - O

We've defined thousands of terms related to health care. This page discusses glossary terms with the letter O.


When people have 20 percent (or more) extra body fat for their age, height, sex, and bone structure. Fat works against the action of insulin. Extra body fat is thought to be a risk factor for diabetes.

Obstruction (ub-STRUK-shun)

A blockage in the GI tract that prevents the flow of liquids or solids.

Occult Bleeding (uh-KULT)

Blood in stool that is not visible to the naked eye. May be a sign of disease such as diverticulosis or colorectal cancer.

Open-label trial

a clinical trial in which doctors and participants know which treatment is being administered to all participants.

Opportunistic infection

an illness caused by an organism that usually does not cause disease in a person with a normal immune system. People with advanced HIV infection suffer opportunistic infections of the lungs, brain, eyes and other organs.

Oral Dissolution Therapy (OR-ul dih-soh-LOO-shun theh-ruh-pee)

A method of dissolving cholesterol gallstones. The patient takes the oral medications chenodiol (KEE-noh-DY-awl) (Chenix) and ursodiol (ERS-oh-DY-awl) (Actigall). These medicines are most often used for people who cannot have an operation.

Oral Hypoglycemic Agents

Pills or capsules that people take to lower the level of glucose (sugar) in the blood. The pills work for some people whose pancreas still makes some insulin. They can help the body in several ways such as causing the cells in the pancreas to release more insulin. Six types of these pills are for sale in the United States. Four, known as "first- generation" drugs, have been in use for some time. Two types, called "second- generation" drugs, have been developed recently. They are stronger than first- generation drugs and have fewer side effects. All oral hypoglycemic agents belong to a class of drugs known as sulfonylureas. Each type of pill is sold under two names: one is the generic name as listed by the Food and Drug Administration; the other is the trade name given by the manufacturer.

Orphan Drug

a drug for which the target population is limited or for which the disease it treats occurs rarely.


OA (also know as degenerative joint disease) primarily affects cartilage within the joints, causing it to fray, wear, ulcerate, and in extreme cases, to wear away entirely, leaving a bone-on-bone joint. At the edges of the joint, bony spurs may form. OA can cause joint pain, loss of function, reduced joint motion, and deformity. Disability results most often from disease in the spine and in the weight-bearing joints (knees and hips).


A thinning of bone tissue caused by gradual bone loss over a long period of time. Osteoporosis produces dry, brittle bones that may easily crack or collapse. Approximately 1 out of every 2 women are at high risk for developing osteoporosis.

Ostomate (AH-stuh-mayt)

A person who has an ostomy. Called ostomist in some countries.

Ostomy (AH-stuh-mee)

An operation that makes it possible for stool to leave the body through an opening made in the abdomen. An ostomy is necessary when part or all of the intestines are removed. Colostomy and ileostomy are types of ostomy.

Oxidative stress

A state characterized by an excess of free radical groups in the body, which creates a potentially unstable cellular environment linked to tissue damage, accelerated aging, and degenerative disease. Oxidative stress can result from many factors, including exposure to alcohol, medications, poor nutrition, trauma, cold, toxins, and over-exercise.

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