Osteoarthritis (OA) is a disease of the entire joint involving the cartilage, joint lining, ligaments, and underlying bone.The breakdown of these tissues eventually leads to pain and joint stiffness. The joints most commonly affected are the knees, hips, and those in the hands and spine.steoarthritis (OA) is a disease of the entire joint involving the cartilage, joint lining, ligaments, and underlying bone.