Turn the key to ignition, ground the terminal on the oil pressure sender, you should see the gauge go to 80+ on the dial. Failure here says the sender wire to the gauge is open or or there is no 6 Volts at the gauge or the gauge is bad (open or mechanically stuck). If the gauge goes to 80 or more, it is possible the sender case is not grounded. To test--check the sender terminal for 6 Volts to battery ground and the case for zero Volts to battery ground. If that test is OK, start the engine and see if there is a pulsing Voltage on the sender terminal. Failure here is either no oil pressure or blocked oil sender passage or an open sender.
Battery ground polarity does not matter, the gauge will read correctly either way if the components are good.
Remember: the oil pressure gauge reads zero with the key off or the engine not running.