Peripheral artery disease (PAD) is plaque buildup in your leg arteries. Your leg arteries carry oxygen and nutrient-rich blood from your heart to your arms and legs. Other names for this are peripheral vascular disease or peripheral arterial disease. Shaped like hollow tubes, arteries have a smooth lining that prevents blood from clotting and promotes steady blood flow. When you have peripheral artery disease, plaque (made of …

Osteoporosis is a disease that weakens your bones. It makes your bones thinner and less dense than they should be. People with osteoporosis are much more likely to experience broken bones (bone fractures). Your bones are usually dense and strong enough to support your weight and absorb most kinds of impacts. As you age, your bones naturally lose some of …

Osteosarcoma is a type of bone cancer. In the beginning, the cancer cells appear to be regular bone cells. Then, they create malignant (cancerous) tumors, and those tumors create irregular, diseased bone. Healthcare providers sometimes call it osteogenic sarcoma. A “sarcoma” is a type of cancer that develops in connective tissue such as bone, cartilage or muscle. “Osteo-” refers to bones. “Osteogenic” means it makes bone cells. Osteosarcoma …

Osteochondritis dissecans (OCD) is a condition that affects your joints. It usually develops in children, teens and young adults. A joint is any place in your body where two bones meet. Osteochondritis dissecans affects the end of one bone in a joint. It cuts off blood supply to a section of the bone which makes it soften and weaken. Eventually, the affected section of …

Orthostatic hypotension is a sudden drop in blood pressure when you stand from a seated or prone (lying down) position. You may feel dizzy or even faint. Orthostatic means an upright posture. Hypotension is low blood pressure. This type of low blood pressure is also called postural hypotension. What is blood pressure? Blood pressure measures the force of blood against artery …

Orchitis (or-KY-tis) is swelling or inflammation of one or both of your testicles (testes). Your testicles are part of your reproductive system — they make reproductive cells (sperm) and testosterone, which is a hormone. Most people assigned male at birth (AMAB) have two testicles. What is the difference between orchitis and epididymitis? Epididymitis is a condition that causes swelling and inflammation in your epididymis. The epididymis is a tube …

Optic neuritis (ON) is a type of neuropathy (nerve disease) that can cause eye pain and vision loss or vision changes. It happens when inflammation affects signals traveling through your optic nerve, which connects your eyes and brain. The cells that make up your optic nerve have a fatty coating called a myelin sheath. When you have ON, that sheath deteriorates. The coating is protective. Without it, the nerve cells can’t …

Oligodendroglioma is a type of brain tumor, but in rare cases, it can form in your spinal cord. These tumors develop from a specific type of glial cell: oligodendrocytes. Glial cells are the support cells of your nervous system. They maintain and protect the neurons that send and relay signals within, to and from your brain. Tumors that come from glial …

Ovarian hyperstimulation syndrome (OHSS) happens when your ovaries swell and leak fluid into your abdomen. OHSS is a complication that typically occurs in people who receive fertility treatments that stimulate their ovaries to produce a large number of eggs. Typically, a person releases one egg per menstrual cycle. People who have trouble getting pregnant may use hormonal medications to help them produce multiple eggs, especially …

Oppositional defiant disorder (ODD) is a behavior condition in which your child displays a continuing pattern of uncooperative, defiant and sometimes hostile behavior toward people in authority. This behavior often disrupts your child’s normal daily functioning, including relationships and activities within their family and at school. It’s common for children — especially those two to three years old and in …