I am thinking faulty gauge, sending unit, or wires. Another reason why I only run mechanical gauges.
You can always pick up a mechanical gauge and hook it up to the block in place of the sending unit, never installing the gauge in the dash. Just sit it on the fender to test your motor. If it test shows good oil pressure, then you know the gauge circuit is faulty. If not, its in the motor. Cheap easy way to eliminate a few things.
Beat it to fit, paint it to match!
Wheel, Wrench, Repeat.