[rockfla]

As Marko39 posted above, get a 1/8NPT brass "T" or 1/8" NPT Male thread "double" female threaded 90 and thread into the rear left top of the motor (found mine on Ebay). Get a mechanical oil pressure gauge (got mine off Amazon) and install that along with the oil sender for your original dash gauge....this will accomplish two things for you.
1) will tell you for sure IF you have oil pressure
2) will allow you to see just how accurate your dash gauge is to the mechanical


I just did this on my 39 Mercury. Not sure just what year you are working on BUT IF you remove your left side floor board of the car it make working in this area WAY easier!!!!!
Once you have your Mechanical gauge installed and see you have pressure on it and none on the dash gauge, then "most" likely you have a bad sending unit. NOT sure IF it is like you gas gauge and can ground it to see IF the gauge is working, hopefully that can either get confirmed or denied. I found my dash gauge is within 2-3 Lbs of my mechanical install!!