ECM/PCM/VCM..."The Computer"...turns on the fuel pump relay when the ignition is turned "on", and holds it for two seconds (perhaps 20 seconds if the vehicle has a "hot fuel module") regardless of oil pressure (which would be zero) or crankshaft-rotation signal.
If The Computer gets a signal from the crank sensor or from the pickup coil, indicating the engine is cranking or running, it keeps the fuel pump relay turned on, or turns it back on.
IF there's a failure anywhere in The Computer, wire harness, or relay, so that the fuel pump relay doesn't supply power to the fuel pump, the oil pressure switch is wired in parallel. Once you get ~4psi of oil pressure, the switch provides a second source of power to the pump.
So the oil switch doesn't have "ultimate say", it's just a redundancy based on a second way of determining crankshaft rotation--because if the engine has oil pressure the oil pump must be turning, and that means the crank is turning.
If everything is working the way it's supposed to, with the engine running the power to the fuel pump would be split between the fuel pump relay and the oil pressure switch based on Ohm's Law and the various resistances in each part of the circuit. The short story is that the relay would provide "about" half the power, and the oil pressure switch would provide "about" half the power. But either one could supply full power if needed due to a failure in the other part of the circuit.