Motor oil instead of antifreeze/coolant

That is going to be very difficult to get back out. If you haven't started it yet, remove the reservoir and clean it out thoroughly. If the engine has been run, you are going to have to drain all the coolant, fill it with water and add some dish washing liquid. Run the engine and flush it thoroughly with water. Repeat if necessary and then refill with coolant. It may have to be repeated a few weeks down the road.