The oil pressue circuit is a simple on/off circuit. It the oil pressure is > 6 PSI, the circuit is grounded. If < 6 PSI, it is open.
If the cluster guage is bouncing, first suspect a weak wiring connection. Is the connector to the oil pressure sender clean and tight?
Since you have changed the pressure sender with no improvement in symptom, either the oil pressure really is messed up or there is a loose connection somewhere.
Start by connecting a temporary jumper from the oil pressure sender harness to a known good ground. This should cause the guage to read OK all of the time. If it still bounces, then the problem is in the cluster.