The purpose of this study is to evaluate the safety and efficacy of GRT6005 compared to placebo in patients with moderate to severe chronic pain due to osteoarthritis (OA) of the knee. This study includes a maximum 21 day Screening Period followed by a 15-week Double-blind Treatment Period and a 4-7 day Safety Follow-up period. Patients who are eligible for the Double-blind Treatment Period will be randomized to one of following treatment groups: GRT6005 high-dose range (400, 600 or 800 mcg), GRT6005 low-dose range (200, 300 or 400 mcg), oxycodone CR (Controlled Release) dose range (10, 20, 30, 40 or 50 mg) or placebo.