I have an 88 ss but I'm going to assume the 83 is similar. On our cars have a oil pressure switch and a oil pressure sender. When oil pressure is above 4 psi the oil pressure swich closes for power to go through. The oil pressure sender is your engine oil pressure and it sends info to the gauge. Me being paranoid the one time I had a funky oil pressure reading I checked oil pressure first I just have a Harbour Freight kit and it has all the fittings https://www.harborfreight.com/engine-oil-pressure-test-kit-62621.html.
My local auto parts ( O'Reilly and Advance) store has one to rent also. After that you can troubleshoot to find if gauge or sender is at fault. All of this is covered in manuals I referenced in other thread.
If you have the sender wire grounded with the key on, it should read zero.
if it does, then most likely the sender has failed, If it doesn't that means that somewhere from the sending unit wiring connector all the way to the gauge, there's a lost/open connection. Has it always done this or just started?
Also using Google to search this topic is usually faster than asking. Here is the search and tons of info on this topic as you scroll down, once again not trying to be a smartass it's just a way to help.