| Word | Description |
| Oral glucose tolerance test | The administration of a sweet liquid solution of glucose to determine how quickly it is cleared from the blood. The glucose is most often given orally. The test may be performed as part of a panel of tests, such as the comprehensive metabolic panel. |
| Osmeterium | Swallowtail caterpillars and their larvae have a gland behind the head which they can project when threatened. This gland has an odor to keep the predators away. |
| Osteoporosis: | Porous bone (osteoporosis) is a disease characterized by low bone mass and structural deterioration of bone tissue, leading to bone fragility and an increased risk of fractures of the hip, spine, and wrist. Men as well as women are affected by osteoporosis, a disease that can be prevented and treated. Osteoporosis can strike at any age. One out of every two women and one in four men over 50 will have an osteoporosis-related fracture in their lifetime. |
| Oxidative stress: | The harmful condition that occurs when there is an excess of free radicals, a decrease in antioxidant levels, or both. |
