My oil pressure gauge spikes immediately to maximum when I start the motor. When I shut the motor off, the needle stays stuck at the maximum reading and when I tap the instrument panel the needle drops back down to 0 bar.

Is it my gauge or my OPRV?

 

Got to be the cluster grounds or Pos terminal on the gauge if it drops when you tap it. Clean all the contacts and I bet it will be fine.

 

Does it peg when you turn the switch to the start position and put voltage on the guages? If so take the wires of the sending unit and see if it still does this.

 

It is mostly likely the oil pressure sending unit. When they go bad, the gauge will stay "pegged" from key on. I'll bet your light at the bottom of the oil pressure gauge works. Replace the sending unit.