where is the OIL PRESSURE SENSOR ON A BUICK RAINER

It is indeed at the front of the engine bay and mounted to the engine block about 6 or 7 inches to the right of the oil filter (looking into the bay). It's buried low and out of sight, just under the exhaust manifold.

A wire that is disconnected from the sender and occasionally bouncing around and making a ground connection would make your gauge act in the fashion you described.

Ok cool how do you get to it to fix it please let me know