Baking fish and meat in the same oven-Part 2-One after the other:
No residue: It is permitted to bake fish in an oven in which one previously baked meat, and it is permitted to bake meat in an oven in which one previously baked fish, so long as the oven is clean of any meat/fish residue, or the food is hermetically covered. This applies even if the oven is Ben Yomo from its previous use. [Some Poskim rule that even those who are stringent to not cook fish in a meat pot, may be lenient regarding this matter to bake fish in a clean meat oven. Some, however are accustomed to be stringent to only bake fish in a Pareve oven. This especially applies if it is difficult to ascertain whether the oven contains any meat residue, in which case it is best to always cook the fish in a Pareve oven, or to cover it.]
There is residue: If one baked uncovered fish in a meat oven that contains meat residue, then the fish is forbidden to be eaten unless it contains 60x the meat reside. The same applies vice versa, that if one baked uncovered meat in an oven that contains fish residue, then the meat is forbidden to be eaten unless it contains 60x the fish reside
How to clean an oven of residue: The only way a proper cleaning can be accomplished is through performing Libun [even Kal] inside of the oven. [This can be accomplished by turning the oven on to a hot temperature for about 15 minutes and having it burn out any leftover residue.] It does not suffice to merely clean the oven with a sponge and water, without also turning it on and burning any leftover residue that is inside.
One may bake fish in a meat oven and vice versa so long as the oven is clean of any previous meat or fish residue [or the meat or fish is hermitically covered]. One is to preheat the oven for about 15 minutes in order to burn any of the residue it contains. One may never bake fish and meat simultaneously in the same oven, and if one does so the food is forbidden unless it was covered.
Those who own a Pareve oven are encouraged to use it for baking fish, rather than use their Fleishig oven, in order to avoid the issue of residue which must be cleaned or destroyed from the oven prior to cooking in it fish or meat.
 See Igros Moshe 1:40; Pischeiy Halacha p. 27; 157; Kinyan Torah 1:24; Minchas Yitzchak 5:20; Hakashrus 1:39-40
Background: As explained in A, modern ovens today contain a Zeiah prohibition. Accordingly, the roof and walls of an oven are saturated with taste of the foods cooked in it and receive a meat/fish status similar to that of a meat/dairy pot. Thus, if one cooks fish in a meat oven or vice versa, it is viewed similar to having cooked fish in a meat pot or vice versa. This is in addition to the possibility of the oven containing meat or dairy residue which can release vapor and enter the food.
 Pesakim Uteshuvos ibid and 92:22 footnote 274; Koveitz Mibeis Levi Inyanei Y.D. p. 41
 The reason: As the danger of eating fish with meat only applies if it contains substance of the meat, and does not apply towards the indirect taste transferred through a pot. [Taz 95:3 “One can learn from here that there is no danger in eating fish that contains the taste of meat through being cooked in a Ben Yomo pot of meat”; Issur Viheter 39:26 “All secretion of taste of meat that is absorbed in a vessel into fish is not Bedieved forbidden due to danger”; Rashal Kol Habasar 9; Kneses Hagedola 116:18; Minchas Yaakov 57:1 concludes “The custom is to be lenient [even initially-Kaf Hachaim 116:20] if the pot is clean”; Peri Megadim 95 M.Z. 3 defends the ruling of the Taz ibid; Lechem Hapanim 116:3; Beis Lechem Yehuda 116:4; Chochmas Adam 68:1; Eidos Biyihosef 2:46; Divrei Hilel 2:32; Daltei Teshuvah 95:5; Ikareiy Hadat Y.D. 14:10; Halef Lecha Shlomo O.C. 312-313; Chinuch Beis Yehuda Y.D. 61; Zechor Leavraham 3:186; Darkei Teshuvah 116:27; Kaf Hachaim 116:14, 16, 20; Sheivet Halevi 6:111 writes it is the custom of the world to be lenient.]
 Pesakim Uteshuvos ibid and 92:22 footnote 274; Hilchos Basar Bechalav 12:50 in name of Rav Klein
 Rav Landau
 The reason: As the residue releases vapor and Reiach which becomes absorbed within the fish and makes it forbidden.
 Michaber 97:2
 Kinyan Torah 1:24; Minchas Yitzchak 5:20; Hakashrus 1:40
 Taz 97:4; Peri Chadash 97:4; Chavas Daas 97:9; Aruch Hashulchan 97:12; Kaf Hachaim 97:34]
The reason: As the fat does not clean off well and requires the oven to be lit in order to burn the fat. [Poskim ibid]