As far as the wires, one wire gets hot when the key is in the on position, one gets (should) hot while the starter cranks (provided the starter was wired up with the wire on the correct studs on the starter solenoid) , and one goes to the + side of the coil (and + side of fuel lock-off). this is a switch, not so much a sender (not a resister to work a gauge). it prevents the engine from running should it ever loose all that oil. usually there is a tee fitting and a separate 1 wire sender for a gauge if it was equipped with a gauge.
you can use an ohm meter to see which connections are a closed circuit at rest then at pressure to see where the wires go on the switch. One should be common to both conditions, it is the one to the coil circuit. the wire that goes hot with the key switch 'on' goes to the one that makes continuity with the common circuit when the switch has pressure, and the one from the side of the starter that gets hot only when the starter turns, goes to the one that has continuity with the common screw when there is no pressure.
my "instant wild as guess" [WAG] would be the wire that should be connected to the starter solenoid is touching the starter or motor and grounding out, and/or is not on the correct post for the small wires (there are 2 small wire posts, one comes from the key switch 'start' side, and pulls in the starter solenoid, and the other has power while the solenoid is pulled in, you should have the wire to the oil pressure switch on the one that gets power -only- _while_ the solenoid is engaged.