• Change from baseline of the average pain intensity over the last week of the Maintenance period at Week 12 or over the entire 12-week Maintenance period, depending on country requirements. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
  

• Incidence rates of treatment-emergent adverse events. Changes from baseline of the Western Ontario MacMaster Questionnaire (WOMAC). Sleep Questionnaire (SQ). 11-point NRS, SF-36, EQ-5D, and others, for a maximum timeframe of 20 weeks. [ Time Frame: 12 weeks ] [ Designated as safety issue: Yes ]
  

This is a randomized (study medication assigned to patients by chance), double-blind (neither patient nor investigator knows which patient gets which study medication, i.e. CG5503, placebo, oxycodone), placebo and active control study. The primary objective is to evaluate the efficacy and safety of orally administered CG5503 prolonged-release (PR) at doses of 100-250 mg (base) twice daily in patients with moderate to severe chronic pain from osteoarthritis (OA) of the knee. The study is being conducted for registration and approval of CG5503 in Europe and outside Europe. The study will consist of five periods: screening (to assess eligibility), washout (3-7 days with determination of a baseline pain intensity), titration (of dose over 3 weeks to the optimal individual level), maintenance (investigational drug intake for 12 weeks with adjustments allowed), and follow-up (2 weeks after end of treatment). The study hypothesis is that the study drug will be more effective than placebo in reducing patients' pain intensity. The secondary objectives include the collection of pharmacokinetic (related to how the body absorbs, distributes, changes and excretes the drug) information for dose verification. The efficacy objectives will be assessed by comparing the baseline pain level to the pain level during the maintainence period. This will be done by looking at the patients' pain diary information (electronic diaries).