When the engine is stopped , the oil pressure goes to zero and the oil runs to the pan. The sensor is now measuring air temp inside the engine . The oil temp is higher than water temp, so the oil sensor reads minimal. On restarting the engine the oil shows warmer again. The oil in the pan will drop from 300-400 degrees quicker than the engine water temp drops from 200. Also the water temp is controlled by a thermostst. The rad may be cool, yet engine at normal operating temp.