A far simpler way is to run the red wire of the buzzer to the alternator warning lamp wire at the alternator, the black wire to the
oil pressure switch wire.
Key on not running, gives you zero volts at the alternator warning wire hence the alt warning lamp is on,
oil buzzer will be off.
Engine running with
oil pressure, 12 volt at the alt warning wire, alt lamp off, no earth at the oil pressure switch, buzzer off.
Engine running no oil pressure, 12 volt at the alt warning wire, earth at the oil pressure switch, buzzer on.