It is common to be faced with questions regarding using an oven for both milchigs and fleishigs. Can I kasher the oven, or should I wait 24 hours? What about just covering the food?
Shulchan Aruch (O.C. 509:5) says that one may kasher a griddle from milchig to fleishig with libun kal, even if it is still a ben yomo. Although kashering a griddle that had been used with issur requires libun chamur, however bliyos of milchigs or fleishigs, since they are heteira (kosher), can be kashered with libun kal.
However, Magen Avrohom 509:11 says that the minhag is not to kasher keilim from milchig to fleishig or vice versa. The concern being that one might forget the change of status and create basar b’chalav.
Nevertheless, the griddle can be kashered, since it self-kashers automatically every time it is used, so there is no concern that one may make a mistake.
Does this leniency apply to our home ovens, which are typically only heated to 350˚F? Furthermore, even if one heated their oven to 550˚F, it takes about an hour to kasher with just hot air. The sevara of Magen Avrohom that it is always automatically kashered, seemingly does not apply to our ovens. Still it seems that the minhag is to be maikel to allow kashering ovens between milchigs and fleishigs, possibly because most people do not have the luxury of having two ovens, plus the consideration that food does not directly touch the walls or racks. The main concern with ovens is zeiya which is questionable.
Standard home ovens are commonly made with metal coated with enamel. It would seem that the enamel coating has the status of glass. The Rema says that the minhag is not to kasher glass. Though some question if this is only a chumra of Pesach, it would seem from Taz (O.C. 87:2) that the chumra of Rema is not limited to Pesach and Rav Belsky has said that one should not differentiate between Pesach, issur or milchigs and fleishigs. However, Magen Avrohom does say that bidieved if one did kasher glass, the food would be permitted. Furthermore, Mishna Berurah 451:137 writes that the minhag was to allow kashering metal pots that were coated with enamel, provided the pot was not used in 24 hours, since it is then ta’am lifgam and a mashehu.
There are many opinions among poskim about how one should deal with milchigs and fleishigs in an oven, and everyone is encouraged to follow the psak of their moreh hora’ah. If one wishes to follow the guidelines of the OU, they should do as follows:
– One should not cook covered milchig and covered fleishig in an oven at the same time. Even though both pans are covered, it is common that one of the covers might open and this will cause a serious shayla.
– If they want to cook covered milchig in a fleishig oven, they should make sure the oven is clean and then burn it out 550˚ F (even ben yomo) for 60 minutes. They may then cook covered milchig in the oven. So long as the cover remained on, the oven need not be kashered again afterwards.
– If they want to cook uncovered milchig in a fleishig oven, they should clean and not cook fleishig for 24 hours. Then they should burn it out 550˚ F for 60 minutes. The same should be repeated before going back to fleishig. Alternatively, they can self-clean the oven, which is libun chamur. Then they need not wait 24 hours.
– One may cook covered pareve in a fleishig ben yomo oven to be eaten with milchig. If they want to cook uncovered pareve in a fleishig oven to eat with milchig, they should make sure the oven is clean. Then they should burn it out 550˚ F (even ben yomo) for 60 minutes. 
 Even if the cover were to open, the food would be permitted, since bi’dieved we rely on the kashering, as per Magen Avrohom.
 This is in accordance with Mishna Berurah that one can kasher lichatchila when it is aino ben yomo and mashehu.
 Even if the cover were to open, the food would anyways be a nat bar nat d’heteira.
 One can rely on the kashering, since the food will anyways be a bar nat d’heteira