72:17 When food is cooked by placing the pot in hot coals (which, obviously, is not our modern practice), one must remove it from the coals before Shabbos begins. If one neglected to do so and then wanted to take the pot on Shabbos, he may not do so because moving the pot would inevitably also move the coals and affect their burning. One may, however, ask a non-Jew to remove the pot from the coals for him.
Please note: the ovens described in the following halacha are not like our modern ovens. Please consult with your rabbi for matters of practical application.
72:18 It is permissible to leave food warming in an oven on Shabbos, even if the opening of the oven is not sealed. One may not, however, open the oven on Friday night, because some pots of food in the oven may not yet be completely cooked. In such a case, opening the oven suspends the cooking process and closing it again causes it to resume. (Rema on OC 249:7 and Mishnah Brurah 249:24 do not prohibit opening such an oven on Friday night, just closing it again.)