In this clinical trial subjects are enrolled, who are suffering from pain due to osteoarthritis of the hip or the knee that is not sufficiently controlled with either a non steroidal anti-inflammatory drug (NSAID) or paracetamol or a weak opioid. This clinical trial tests the pain relieving effectiveness of OROS hydromorphone, a once-daily formulation of a strong opioid against placebo in patients, who previously did not receive any strong opioids. The drug class of opioid analgesics can broadly be classified into strong and weak. Weak opioids (for example tramadol, codeine, dihydrocodeine and tilidine) are useful for mild to moderate pain and the strong opioids (for example morphine, fentanyl and hydromorphone) are useful for moderate to severe pain of different origin. OROS hydromorphone is an opioid, which is available in a prolonged-release tablet in different dosage strengths. The primary aim of the study is to test the efficacy of OROS hydromorphone against placebo at an individual dose sufficient to control the pain and to establish the usefulness of a new low-dose formulation of OROS hydromorphone (4 mg hydromorphone per tablet) for initiating the treatment and for dose titration. The clinical trial tests the effect of the treatment on symptoms of pain, stiffness and physical function. The effect of the treatment on parameters on health related quality of life as well as quality of sleep will be measured. The safety of the treatment will be recorded by measuring blood pressure, heart rate, and respiratory rate.This clinical trial is a placebo-controlled trial, meaning that one group of patients will receive the drug to be tested (OROS hydromorphone) while the control group receives an optically identical tablet with no active ingredient, a so-called placebo. A total number of 270 patients will be enrolled in this clinical trial and assigned to one of two treatment arms at an equal ratio (i.e. 135 patients per treatment). Patients will be randomly assigned to one of the two treatment arms, like flipping a coin to decide which treatment they will receive. Neither the patient nor the doctor will know to which of the two treatment arms the patient is assigned to and neither the patient nor the doctor can influence the assignment to the treatment arm. During the whole treatment period paracetamol will be allowed to be taken as needed in case of pain. Medical history and physical exam will be conducted during the screening visit, followed in 1 week by the baseline visit where after completing several questionnaires assessing pain, physical functioning quality of life and sleep quality, the patient will be assigned to one of two treatment groups. After starting the study treatment the patient will visit the doctor 7 times: at week 1, 2, 3, 4, 8, 12, 16 and at a follow-up visit after the end of the treatment period at week 16. At week 16 questionnaires will again be completed and the results will be compared to the baseline findings. 4, 8, 12, 16, 24 or 32 mg of OROS hydromorphone tablets or matching placebo tablets taken for 16 weeks. All tablets are taken by mouth at the same time each day in the morning. Tablets have to be swallowed whole without chewing or crushing. After completion of the treatment duration (or at early withdrawal), the study medication is gradually tapered down over a maximum of 6 days.