Osteoarthritis, also referred to as OA, is the most commonly occurring type of arthritis. According to the Arthritis Foundation, over 30 million Americans have been diagnosed with OA. It has a gradual onset and generally develops around the age of 50 and up. OA is a disease of the joints, affecting the entire joint and involving the cartilage's deterioration. This results in bones (such as the knees, hips, back) rubbing directly against each other.