This is a multi-center, randomized, double-blind, double-dummy, parallel, placebo- and positive-control (naproxen), dose-ranging study to assess the efficacy and safety of 3 oral doses of PLA-695 administered QD and 1 dose of PLA-695 administered BID for 6 weeks. Subjects will be randomized to 1 of 6 treatment groups: PLA-695 50 mg, 200 mg, or 400 mg QD, PLA-695 200 mg BID, placebo, or naproxen 500 mg BID. Subjects will undergo a washout period of 2 to 14 days. The number of days for washout will be determined by the pre-study NSAID of each subject. Subjects who washout from previous therapy with an increase in signs and symptoms of OA will be randomly assigned to receive PLA-695.