http://www.bureauveritas.de/wps/wcm/connect/bv_de/local/services+sheet/fs-cer-ohsas-18001?presentationtemplate=bv_master_v2/Services_sheet_full_story_presentation_v2

[...] Wir sind bereits nach OHSAS 18001:1999 zertifiziert. Bis wann muss eine Umstellung erfolgen?

Es wurde eine Übergangsfrist von knapp 2 Jahren nach Veröffentlichung festgelegt. Als Stichtag zur endgültigen Umstellung auf die neue OHSAS 18001:2007 wurde der 01.07.2009 festgelegt. [...]

 
http://www.tuev-sued.de/management_systems/safety_management_certification/ohsas/revision_ohsas_180012007 (not online anymore)

Revision of the OHSAS 18001:2007
The occupational health and safety management system standard OHSAS 18001:2007 was published in a revised version by the British Standard Institution (BSI) in July 2007. A number of international certification and standards bodies contributed to the development of the review.

Transition period
BSI set a period of two years for the implementation of the revised norm. Therefore certificates to the OHSAS 18001:1999 will become invalid at the latest by 30th June 2009. It is however recommended that the amendments and additional requirements be implemented as soon as possible in the certified company to ensure that maximum benefit is attained from the improved standard.

The key changes
As well as improvements to the general clarity of the document, the new version places greater emphasis on health and there is a significant improvement in the alignment with the ISO 9001:2000 and the ISO 14001:2004.

Summary of key changes between the BS OHSAS 18001:2007 and OHSAS 18001:1999 [...]