Hi John
Can you scope the oil pressure switch on a long road test. It may flicker to earth not bringing the light on [ If you can catch it with light on even better]

If the light comes on and the scope doesn't go to earth you can be happy its not oil pressure.

Then switch disconnected, wire a fused bulb to the switch and scope again. If it flickers to earth you know the faults the oil pressure or new switch.

Hope that makes sense regards Ian.