Put it all back together. Go out and find the oil pressure sending unit. It's location will vary depending on what engine you have. If you have a 302 or 351w, it will be down near the oil filter and fuel pump.

Pull the wire off the sending unit. With the key in run, engine off, put the sending unit wire on to a good ground. The oil pressure gauge should swing full scale either high or low. Then take the wire off ground and let it hang in the air, touching nothing. The gauge should swing full scale the opposite direction. If it passes this test, then the wire and the gauge are good, and there is a problem with the sending unit.

This test will work for all the gauges. For the gas gauge, simply pull the wires off the tank, ground and un-ground the wire and the fuel gauge will swing back and forth if all is good with the gauge and the wire to the gauge. Same for the temp gauge also. It has a sending unit on the engine like the oil pressure gauge.