Is it kosher to use one oven for both meat and milk dishes cooked separately. For instance, can I cook a meat dish and, after it's done, put in a milk dish and cook that?
Or does the oven have to be either meat or milk permanently?
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This is not ideal but is the reality for some. There are plenty of discussions by distinguished authors on the halachic perspectives (e.g., R Doniel Neustadt here, OU Kosher here, R Michael Broyde here and R Chaim Jachter here) but, in practice, the following two answers give you practical guidelines
How do I bake dairy as the oven is mainly used for meat?
It's pretty simple. Any dairy you bake in that oven has to be wrapped in a double layer. Aluminum foil is great. Make sure each wrapping is complete, covers the food entirely, and goes all the way around the entire pan or pot—not just the opening. The food and your oven will both be completely kosher.
A more complete answer from aish.com
The best, if possible, would be to have two separate ovens – or a double-chambered oven – one for meat and one for dairy, since mix-ups are so common. However, if this isn’t possible for you, it’s fine to use a single oven for both types, so long as you observe the following guidelines:
(a) You should determine which type of food you cook most often, whether meat or dairy. That type you can cook as normal in both covered and uncovered pots and pans.
(b) The opposite type should only be cooked in a covered pan or pot. If the opposite type is completely dry, then there is no need to cover it (but the other requirements below do apply).
(c) Separate racks should be used for meat and for dairy (or a layer of aluminum foil should be placed on the rack underneath the opposite-type pan).
(d) The oven should be cleaned out from any liquidy spills of gravy or grease in between uses of meat and dairy.
In terms of pareve, that may be cooked as regular in an oven, even on the same racks, with the following restrictions. (For simplicity, I assume the oven is regularly used for meat.)
(a) The oven must be clean of any meaty spills. If the oven was not checked and cleaned beforehand, the pareve may not be eaten together with dairy, but it may be eaten immediately before or after (even in the same meal, but the hands should be washed before switching).
(b) Even if the oven is clean, if it was used to cook liquidy uncovered meat within the past 24 hours and the pareve is also liquidy and uncovered, then it may not be eaten together with dairy but can be right before or after.
(c) If the oven is clean and has not been used within 24 hours with liquidy uncovered meat, liquidy pareve foods should not be cooked in it in order to eat them together with dairy, but if they were cooked, they may be eaten together with dairy.
Note that Sephardic Jews are more lenient with such indirect tastes of meat and milk (based on Shulchan Aruch Y.D. 95:1-2). Thus, they may bake pareve foods in a dairy/meaty oven without restriction and the food is still considered pareve.
(Sources: Igrot Moshe Y.D. I 59, Yabia Omer Y.D. V 7, Yalkut Yosef 89:35, The Laws of Kashrus by R. B. Forst pp. 226-228.)
The short answer is yes, if you take the right precautions. The exact parameters are going to depend on which posek you ask, but this is the Star K's recommendation.