Leaving a Regular Oven On for Shabbat at 150 Degrees Without Touching It

To the honored questioner, It is permitted to leave the oven on. However, it is not permitted to place any food on it on Shabbat. This is because of the rabbinic decree of mefzi kemevashel, "it appears as if one is cooking" - meaning that even when no actual cooking can apply to the food, such as a dry food that was already fully cooked, it is forbidden to place it on the fire during Shabbat, since it appears to others as though one is now beginning to cook. Leaving food inside the oven from before Shabbat is only relevant when there is some visible sign that one will not come to touch the knobs that change the size of the flame. One must place a cover over them, and then it is permitted to leave only food that was already completely cooked before Shabbat. 

With blessings, 

Rabbi Nachum