Remove the 1 wire on the sending switch, see what needle shows, key on and key off.
Then ground that 1 wire and see what needle shows.
Ford uses a switch for oil pressure, so it is either on or off, so 0 or 1/2 on the gauge.
On most this switch is the ground for the gauge, when engine is off there is no ground, switch is open, so gauge shows 0, when engine starts and pressure is above 5 psi the switch closes and gauge is now grounded so shows 1/2 on the needle.
On others the switch is closed when engine is off, this turns on the oil light, when engine starts and oil pressure is above 5psi the switch opens and oil light goes off.
So what you are seeing on the gauge is electrical signals not actual oil pressure.
So most likely a wiring or sender issue.