There is a test terminal on the ignition switch that is wired to the oil light same as the engine sensor. The ignition switch is supposed to test the oil light whenever the switch goes into the START position. One check would be to unplug the ignition switch then check the oil sensor lead at the engine. An ohmmeter will tell if the short is still present. If it is, one of those two wires going to the oil light is shorted to ground somewhere. If the short clears, the problem is in the ignition switch.
The power seat (and power windows if present) get fed by a circuit breaker on the fuse block. A good test would be to see if there is 12V power at the circuit breaker output by using a test lamp or voltmeter. Also do power windows work? If a short to ground is present in one these systems the breaker will cycle on and off. Breaker should be warm if it is cycling on and off.
If power is present at the breaker output, could just be the seat switch has become unplugged. Running the seat all the way forward should not harm anything as long as components are in good working order.
At least these ideas should provide a good start.
What has been, can be again. (Bob Wills, 1942)