Add a fuse, a relay to the ignition power on, a toggle switch for Gas on & a momentary pushbutton for priming.
Prime PB will let you run the pump before ignition is on & only run when you press the PB. The pump will run when the "Gas On" switch is on, the ignition is on & the Oil Pressure switch is on.
This will run like Bob's system except there is another PB to prime the system.