Stage 3 OA is classified as “moderate” OA. The cartilage between bones is showing obvious damage, and the space between the bones is narrowing. People with stage 3 OA of the knee are likely experiencing frequent pain when walking, running, bending, or kneeling. They also may experience joint stiffness after sitting for long periods of time or when waking up in the morning. Joint swelling may be present after extended periods of motion, too.