This video link may be useful in which BSI explain how BS OHSAS 18001 was developed, the background to moving from BS OHSAS 18001 to ISO 45001 and how clients can manage the transition when the new standard is published in 2016

http://www.youtube.com/attribution_link?a=eD3sgkZTnaQ&u=/watch%3Fv%3D8i0fIhoLDOA%26feature%3Dem-share_video_user