When is the oil light supposed to come on?

And how can I test it to know its working?

Will a leak in pressure cause it to not work?(I read they act on pressure, and its a pressure sensor so...)..my light isn't working, but I decided to test my tighten'ed e brake around a turn and it caused the oil light to flicker on. It also did it when I released the clutch all of a sudden. I'm guessing the little ball in the sensor may be loose or something, but I doesn't happen when I go WOT or nothing. I also just changed my oil and there was some plastic stuff or something blocking part of my filler tube.

 

the light got to come when there is not oil pressure, the oil sensor may be death so remplace it, the light have to turn on when the car its turned off