Osteoarthritis (OA) is the most common form of arthritis, typically affecting older adults over age 45. It occurs when the cartilage between bones breaks down, causing pain, stiffness, and reduced mobility. Risk factors include age, female sex, joint injuries, obesity, genetics, and overuse. Symptoms include joint pain, stiffness, swelling, and crepitus. Diagnosis is made through physical exam, x-rays showing joint space narrowing and bone spurs, and ruling out other causes. Treatment focuses on reducing symptoms through medications, exercises, weight loss, bracing, and joint replacements for severe cases.