Unplug the wire going to the oil pressure sender, test resistance between
the sensor and ground; should be zero with engine off and very high with
engine running; if not, faulty sensor or engine ground.  Check for voltage
at the sending wire you pulled off of the sensor; I think it's supposed to
be 5v with the key "on"; if no voltage, look for a poor connection at the
wiring harness in the instrument cluster.  Make sure that the connector at
the oil pressure sensor is clean and securely attached when you plug it
back in.  That should cover the simplest and most likely problems.

--
Brett in Portland, OR
"Albert" '82 VanaFox I4 Riviera