Oxaprozin is used to relieve pain, tenderness, swelling, and stiffness caused by osteoarthritis (arthritis caused by a breakdown of the lining of the joints) and rheumatoid arthritis (arthritis caused by swelling of the lining of the joints). Oxaprozin is also used to relieve pain, tenderness, swelling, and stiffness caused by juvenile rheumatoid arthritis in children 6 years of age and older. Oxaprozin is in a class of medications called nonsteroidal anti-inflammatory medications (NSAIDs).