98 jeep cherokee oil pressure.

I just got a Jeep. I needed to change the radiator so I done that. But before I done this i checked my oil level just because I thought I might as well since I was there, saw that I needed some so I kept that in mind. So I just got done installing new radiator and filling with new coolant. Took for a test drive and noticed the oil was fluctuating, goes up to normal when stepping on gas, goes down to first tick when idling. Not sure right now if it is because of low oil or if I done something wrong with coolant. Before I done all this the pressure gauge read normal. Not sure if this is normal.