Toyota corolla 2000 low oil pressure and indicator light on.

The mechanic troubleshoot it by testing it and observing it at the same time. Apart from oil pump, is there any other possible area that can be blocked?

Thanks for the response # Alex. Yes the mechanic observed it with the manual process and opened the top down when starting the car. The oil could not spill out the way it use to be before. On last Saturday (on the 14th) a pump he used supplied oil but yesterday after changing a failing camshaft (that has been welded at the joints to permanent) the oil is no longer coming out through the engine top opening were the camshaft is located. Andy