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 controlled release (CR) dose range (10, 20, 30, 40 or 50 mg) or placebo.

Inclusion Criteria:

• Diagnosis of osteoarthritis (OA) of the knee
• OA knee pain present for at least 3 months
• OA knee pain is the predominant (ie, most painful) pain condition
• Patients require medication on at least 4 of 7 days per week to treat OA knee pain for at least 1 month.
• Dissatisfaction with current OA knee pain treatment and willingness to stop taking all pain medications except for the study drug and allowed rescue medication
• Average daily pain rating of at least 5 on a 0 to 10 point numerical rating scale during the 1 week prior to randomization
• Female patients either postmenopausal, surgically sterile or practicing a medically acceptable method of contraception
• Male patients either status post-bilateral vasectomy or using barrier contraception

Exclusion Criteria:

• Knee pain due to a disorder other than OA
• Other pain that can confound the assessment of, or contribute to, pain at the reference knee
• Surgery at the reference knee within 6 months of Screening or a history of joint replacement surgery at the reference knee
• Trauma to the reference knee within 6 months of Screening with active symptoms
• Steroid injections in the reference knee within 3 months of Screening
• Hyaluronic acid injections in the reference knee within 6 months of Screening
• Body Mass Index > 40 kg/m2
• Patient with any protocol-excluded or clinically significant medical, surgical, or medication history that would limit the patient's ability to complete or participate in this clinical trial or could confound the study assessments