A relay is just an electrically activated switch in simple terms. You apply a voltage (12 or 24vDC for automotive applications) to two terminals (the coil) and it will "relay" whatever is connected to the input terminal out through the output terminal.
In your case, you'll apply the 12v from the oil pressure switch to turn it on and it'll relay whatever high current feed you rig up on to your fuel pump.
finding a live connection only on with ignition???
Moderator: RichardW