obesity A body weight 20 percent or more above the accepted standard for a person’s age, sex, and body type.
occipital lobes The back part of the brain.
occlusion Blockage of an opening or vessel in the body.
open-heart surgery A major surgical procedure on the heart during which circulatory functions are temporarily taken over by a heart-lung machine.
orthopnea Shortness of breath that occurs when lying down, usually a symptom of heart failure.
orthostatic hypotension A sudden drop in blood pressure that occurs when a person stands up. Can cause fainting.
ostium primum In the heart of a fetus, an opening that serves as a link between the two developing atria. It closes as the septum fully develops.
ostium secundum In the heart of a fetus, an opening that develops as the ostium primurn closes, continuing communication between the atria.