What is Daypro (oxaprozin)? Oxaprozin is a nonsteroidal anti-inflammatory drug (NSAID). Oxaprozin works by reducing hormones that cause inflammation and pain in the body. Oxaprozin is used to treat pain or inflammation caused by osteoarthritis or rheumatoid arthritis in adults. Oxaprozin is also used to treat rheumatoid arthritis in children who are at least 6 years old. Oxaprozin may also be used for purposes not listed in this medication guide.