This will be a randomized, double blind, placebo controlled, parallel group, multi-center study in subjects with moderate-severe OA. Subjects will be randomized to receive flavocoxid 250 mg bid, flavocoxid 500 mg bid, naproxen 500 mg bid or placebo in a 2:2:2:1 ratio. Subjects will take study materials twice daily for twelve (12) weeks. After the 12 week visit subjects in the placebo arm will be re-randomized to the two flavocoxid arms. All subjects will then continue on the study for an additional 12 weeks.