Subjects with moderate to severe pain due to osteoarthritis (OA) will be randomised to oxycodone/naloxone prolonged release (OXN PR) or oxycodone prolonged release tablets (OxyPR) alone to demonstrate that the treatment with OXN PR tablets is non-inferior to the treatment with OxyPR with regards to analgesic efficacy and locomotor function and to demonstrate that subjects taking OXN PR have improvement in symptoms of constipation compared to subjects taking OxyPR alone.