What should I do about osteoarthritis in my knee?

Depends. It depends on what has already been done, the advancement of it, the degree of discomfort you are experiencing. You will need to modify the way you walk, starting with the shoes you wear. You may need a knee brace to improve mechanics of it. Tylenol, (acetaminophen) anti-inflammatory meds, glucosamine may be something to start. Physical therapy, steroid and hyaluronic acid injections is another option.
Many otions. Treatment options include bracing, injection, pain medication, anti-inflammatory medication, physical therapy, and surgery.