^^ditto. Check that oil, top if off, fix the leak, put in a real oil pressure gauge to test the actual engine pressure(even just temp, this would be a good idea). How many miles on the engine?
But yeah, it could be literally insufficient oil, oil pump, bad gauge cluster, bad oil pressure sending unit, etc.
When RPMs are low, oil pressures are low. When RPM rises, oil pressure rises.
'01 XLT disguised as a '03 Fx4 Level II.