Remember the oil temp is directly related to coolant temperature. In most engine designs, the oil runs about 20F/10C hotter than the coolant.

Now if you fit an oil cooler this changes, but the coolant temp still affects it. The heat has to go somewhere, and it won't wait until it gets to the radiator/cooler. Heat transfers anywhere it can: through the alumi head, valve stems, cylinder walls, etc. And if the coolant is hot, heat will flow from it into the oil passages.

So if you keep your coolant temp stable, it helps keep oil temp stable too.