Possibly the oil pressure sending unit (that operates the gauge) or the gauge unit itself, which is the entire "circle" of gauges, ie the fuel gauge, temp gauge, and oil pressure gauge (and the economy gauge if your cluster has it).
It would be pretty easy to determine which is the most likely culprit by removing the wire from the sender unit (near the oil filter housing) and observe the gauge reading with the key "on", then connect the disconnected wire to a good ground and see what the gauge reads then. Should go from one extreme to the other (sorry don't remember which reads pegged high and pegged low, but you'll figure that out. If the gauge doesn't move at all during this test, assume the gauge assembly or wiring is bad, maybe just a bad connection at the back of the instrument panel.
If it's the gauge unit itself, they are available as a seperate part.