606. Air Getting in the Oven

Shabbos 3:14

Whenever something would be spoiled by air getting into the oven, we do not prohibit heating it in an oven because there is no danger that one will open the oven to stoke the coals. This is why we may put flax into an oven before Shabbos because if the oven is opened, the flax will be ruined.

Shabbos 3:15

If one put a whole kid in the oven, the rules are the same as for meat from a goat or an ox: it is prohibited out of concern that one may come to stoke the coals unless the oven is sealed. However, it is permitted to hang the Passover lamb in an oven immediately before nightfall even if it is not sealed. This is because the members of the group that has gathered to share the sacrifice are careful about this.