you are sure it is oil and not transmission fluid? The only place the oil and coolant really share areas with the opportunity to mix is the head gasket. Typically which ever one leaks that is closest to the cylinder gets pushed into the chamber farther away. (if the cylinder is leaking as well). You could do a compression test and a cylinder leak down test to check for head gasket leaks. Even this won't be 100% fool proof if it is a minor leak.