On a 2002 Jeep Liberty (and most cars), the oil pressure light coming on briefly when you turn the key to the "on" position (but before starting the engine) is
normal. It's a self-test to make sure the light bulb itself is working. The light should go out once you start the engine and the oil pump begins pressurizing the oil system.
If the light stays on after starting the engine, that's a serious problem indicating low oil pressure, which can cause catastrophic engine damage if driven. You should immediately turn off the engine and check your oil level. Low oil pressure can be due to low oil, a failing oil pump, or other engine issues.
