To demonstrate that in a group of opioid naïve non-constipated subjects with non malignant pain treatment with OXN tablets leads to a higher responder rate compared to MOR PR. A responder is defined as a subject who meets the following criteria: A subject, whose bowel function improved, did not change or did not have an unacceptable worsening compared to pre randomisation (subjective evaluation). Subjective evaluation: To what extent did your bowel function (e.g. frequency of defecation, stool consistency, ease of defecation, painful defecation) change during the treatment with study medication: Bowel function is substantially improved; Bowel function is slightly improved; Bowel function is unchanged; Bowel function is slightly impaired, but still acceptable; Bowel function is substantially impaired, no longer acceptable A subject who does not discontinue from the study due to an AE of constipation during the first 14 +/- 2 days of the double-blind phase.