What causes the oil pressure light to stay on while driving? on 1995 Mercury Mystique

There is plenty of oil by the dipstick. I had just taken the car in to see why it was stalling out in traffic, and they said it was the idle speed control and needed throttle body service. As soon as I drove it away, it stalled twice in traffic within 10 minutes, plus now it has this alarming clack-clack-clack sound while driving and idling, and also the oil pressure light now stays on almost constantly (it didn't before the "repair")!

Asked by for the 1995 Mercury Mystique
Yikes! maybe you need to have the oil pressure checked