So an accurate diagnosis is important. Osteoarthritis is the most common form of arthritis. Rheumatoid arthritis affects about one-tenth as many people as osteoarthritis. The main difference between osteoarthritis and rheumatoid arthritis is the cause behind the joint symptoms.Osteoarthritis is caused by mechanical wear and tear on joints. Rheumatoid arthritis is an autoimmune disease in which the body's own immune system attacks the body's joints.heumatoid arthritis affects about one-tenth as many people as osteoarthritis. The main difference between osteoarthritis and rheumatoid arthritis is the cause behind the joint symptoms.