VW uses 2 oil pressure switch. a low, 0.3 bar and a high 1.8 bar swicth. the low is normally always ground. when you start the car or idle, if there is enough pressure (only low pressure is needed) then the pressure causes a break in the ground and the buzzer NOT to go off. if your buzzer goes off when starting or idling... you're not even building .3 bar of pressure.
the second switch is normally not grounded...only activates in between 2k-2.5k rpm...if, while within this rpm range, your car is unable to produce about 1.8 bar of oil pressure, then the switch does not activate and the ground is not completed...causing the oil buzzer to go off."