Newer Jeeps like probably 2001 and newer don't directly indicate oil pressure on the gauge... the computer just gives a mid-scale reading for all pressures except when it's way too high or way too low when it will give a "Check Gauges" indication and give that extreme high or low reading. When the computer is happy with the true oil pressure, the oil pressure signal it sends to the gauge won't vary in step with engine rpms and it doesn't need to actually get any oil pressure at first to show a mid-scale "ok" oil pressure reading before the engine starts. That is to keep the less astute people from unnecessarily worrying and pestering their Jeep dealers when they wonder why their oil pressure "goes up and down" (as it should).
Older TJs like mine acually give a true oil pressure reading that rises and falls as it should in step with engine rpms. Mine sits at zero for a second or two after starting the engine and then it jumps up to where it should be. This is probably true for '97 to perhaps the 2000 or 2001 model year, it's after that where the gauge was reprogrammed to not show a varying oil pressure if everything is within normal parameters.