What is Osteoarthritis? Osteoarthritis is the most common form of arthritis that most often affects middle-age to elderly people. It occurs when the protective cartilage on the ends of your bones wears down over time causing pain and stiffness in joints. Osteoarthritis is a disease of the entire joint, involving the cartilage, joint lining, ligaments, and bone. Although osteoarthritis can damage any joint in your body, the disorder most commonly affects joints in your hands,