I agree with Hank, it sounds like an electrical issue. I think that light switch is a ground turn on so you maybe have something giving it some ground (possibly the switch internally). This can happen when a sensor is installed in place of a switch, there should be a oil pressure switch and a oil pressure sensor depending on your model Montero. The switch controls the light and the sensor is the sender for the gauge if equipped, it may be a combination switch/sender that does both jobs I don't know because i haven't looked it up for you and my gen 1 wasn't equipped with gauges from the factory so i'm not readily familiar. Do a little research and i think you'll have this sorted in short order.

1999 Montero SAS'd on Kings and stuff

1998 Montero trying to get a V8 Swapped