When I put the 6 dial dash in my Nova I had to wire up the Oil pressue gauge.
I had to get a oil pressure sensor, I got mine from the breakers off a Cavalier SRi130 for £2. This has 2 terminals on it. One goes to the light, the wire that is already connected to the switch. I then had to run a new wire through the car to behind the oil pressure gauge.
On my Nova, it was near the bottom pulley of the engine, at the back of the engine, quite low down. (if that makes sense)
You should be to trace the wire to a terminal on the end of a switch. (about 5cms long) If you unbolt this using about 13mm spanner and switch it for the new oil pressure sender. The sender unit is bigger than the original switch.
Not sure what terminal is the warning light and which one is the pressure though.