This is a multicenter, outpatient, multiple-dose, placebo- and active controlled, double-blind, double-dummy parallel group, randomized (stratified by unilateral vs. bilateral knee OA) trial. The trial will enroll subjects who have been diagnosed with OA of the knee (confirmed by weight-bearing x-ray) and whose condition is in flare at baseline. Subjects who qualify to enter the study will be randomized to topical TDS-943 40 mg bid, oral celecoxib 200 mg qd, or placebo in a 3:2:2 ratio.