Right now, as of Summer 2008, yes they are declining with more and more companies cutting hours and facing downsizing due to recession. There are alot of agencies that help fill the needs for OHN's on a contract, either long or short,or episodic.
Most OHN jobs are where there is a concentration of businesses in an area, usually in metropolitan areas where there is a larger population needing the services or a large company that serves many companies. Smaller companies outsource to Occupational Health companies that provide services either off site or onsite.
(examples: Medcor, US HealhWorks, Concentra, CHD Meridian, CHS, YOH Healthcare, etc.)