Please note: the ovens described in the following halachos are not like our modern ovens. Please consult with your rabbi for matters of practical application.
72:19 While a person may warm food in an unsealed oven on Shabbos, this applies only when the food was already warmed for a long time before Shabbos began. In this way, the food will be cooked enough before Shabbos so that it could be eaten in a pinch (i.e., one-half or one-third cooked - Mishnah Brurah 253:38). If one wants to start warming food very close to the start of Shabbos, he must then seal the oven. If one neglected to do this (even accidentally - OC 254:8), then the food is forbidden until one waits after Shabbos the amount of time it would take to cook this food.
72:20 At a time when ovens were sealed shut, if a person wanted to open one on Shabbos, it was best to have it opened by a non-Jew. If a non-Jew was unavailable, the next-best course of action was to have a child do it. If no child was available, then an adult was permitted to open it, albeit in a manner different from the way in which one might do so on weekdays. (All this is because the act of unsealing the oven resembles the melacha of soseir - demolishing - Mishnah Brurah 259:22. If he knows that coals are still burning in the oven, then a Jew may not open it regardless of whether or not it has been sealed - MB 259:21.)