There are many differences, but the main change is that ISO 45001 concentrates on the interaction between an organization and its business environment while OHSAS 18001 was focused on managing OH&S hazards and other internal issues. But the standards also diverge in many other ways:
ISO 45001 is process-based – OHSAS 18001 is procedure-based
This standard replaces OHSAS 18001:2007, an OH&S management system standard developed by a smaller group of international experts outside of ISO.
ISO 45001 is dynamic in all clauses – OHSAS 18001 is not