Oil Pressure Sensor?

Start with the cheap stuff first...

The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C. Clean the engine side and the firewall side with some fine sandpaper so that you have clean, shiny bare metal to connect to.

Replace the stock oil pressure sender: it is located down by the oil filter on the front of the engine. cost is $11-$15

Stock gauges are notoriously inaccurate, and many guy report low oil pressure with them. A mechanical gauge under the hood or in the car is a good investment.

Mechanical oil pressure gauge installation..
1.) Remove old pressure sender. It is located down by the oil filter and has 1 wire on a push on screw connector.
2.) Install ¼” pipe tee fitting with a short ¼” nipple on the place where you removed the oil pressure sender.
3.) Install the original oil pressure sender back in one of the tee ports.
4.) Install the gauge line restrictor in the remaining port of the tee.
5.) Connect the flex tubing or hydraulic hose to the gauge restrictor. Be sure to route the tubing or hose away from the exhaust manifolds. Be sure to either use hydraulic hose or oil and heat resistant flex tubing for the pressure line. Do not use copper tubing for the connection to the gauge restrictor or engine. Over a period of time, the vibration will work harden the tubing and cause it to crack and fail.
6.) If the gauge is going to mount under the hood, connect it to the end of the hose or flex tubing. Use some Tie-wraps to secure the gauge and its plumbing to the wiring harness on the driver’s side inner fender.
7.) If the gauge is going to mount in the passenger compartment you get to find a place to run the tubing through the firewall. I suggest that you pick your spot from the inside since that is the area with the most difficult access. Do not run the tubing through the exact same hole as the steering shaft. It will tangle up with the steering shaft and cause major problems.
8.) Install the gauge inside the car in the location of your choosing. Connect the pressure line to the gauge and secure it out of the way of moving parts with Tie-wraps.
9.) Be sure to ground the gauge lighting wire to clean, shiny bare metal. Connect the other wire from the gauge lighting to a light blue/red wire on the radio wiring harness. There are 2 connectors in the radio wiring harness, but only one of them has the light blue/red wire you need for gauge illumination. Solder the gauge lighting wire to the light blue/red wire on the radio harness and cover the soldered joint with heat shrink.

How to solder like a pro - Ford Fuel Injection » How To Solder Like a Pro a must read for any automotive wiring job.

Be sure to use a gauge restrictor fitting in the pressure line going to the gauge. The restrictor will prevent all of the engine's oil supply from ending up on the pavement before you can stop it if the gauge line fails. I had a gauge line rupture, and the restrictor fitting saved my engine from serious damage.

Buy an autometer mechanical pressure gauge. Stock gauges arent very accurate. Its almost never the pump, so cross that thought off your list.

edit: jricker beat me to it.
I prefer to T the fitting so stock and mechanical gauges work.