CG5503 is a centrally active pain-relieving drug being investigated for the treatment of acute and chronic pain. This study is designed to assess the safety of CG5503 Immediate Release (IR) in men and women who are 18 years of age or older and who have had chronic (for at least 3 months) low-back pain or chronic pain from osteoarthritis of the hip or knee, and who require daily analgesic medication. In this double blind study (neither patients nor investigators will know what treatment is given), patients will be randomly (patients are assigned different treatments based on chance) assigned to receive either CG5503 base IR or oxycodone IR to begin a 90 day treatment during which each patient will return to the medical facility every 2 weeks. For the CG5503 IR group, patients will take 50 or 100 mg orally every 4 to 6 hours as needed to a maximum dose of 600 mg per day. Oxycodone IR patients will take 10 or 15 mg orally every 4 to 6 hours as needed to a maximum dose of 90 mg per day. Patients may continue taking their regular, stable, non-opioid medication throughout the study. The effectiveness of study treatment (CG5503 base IR or oxycodone) will be assessed with 11-Point Numerical Rating Scale and Patient Global Impression of Change. Using these scales, patients will indicate their pain intensity level and overall status of their well-being. Opioid withdrawal symptoms of CG5503 IR dose of 50 or 100 mg will be assessed with clinical opioid withdrawal scale [Subjective Opiate Withdrawal Scale (SOWS) and Clinical Opiate Withdrawal Scale (COWS)] at the end of treatment. Using these scales, patients will indicate the physical components of drug withdrawal and the severity of symptoms associated with rapid withdrawal. Safety will be assessed using physical examination, monitoring of adverse events, clinical and laboratory measures, and 12 lead electrocardiogram (ECG) results. Constipation, vomiting, and sleep assessments will be performed. No formal hypothesis testing will be done in this study. For the CG5503 IR group, patients will take a flexible dose of 50 or 100 mg orally every 4 to 6 hours as needed to a maximum dose of 600 mg per day for 90 days. Oxycodone IR patients will take a flexible dose of 10 or 15 mg orally every 4 to 6 hours as needed to a maximum dose of 90 mg per day for 90 days.