ריחא מילתא or לאו מילתא, the large open oven, one of them covered, a דבר חריף (sharp food), the same pan (מחבת), 60 against the prohibition, hot bread on a barrel of יין נסך, and smelling a prohibition (Shulchan Aruch, Yoreh De'ah 108 — 7 se'ifim)
אֵין צוֹלִין בָּשָׂר כְּשֵׁרָה עִם בְּשַׂר נְבֵלָה אוֹ שֶׁל בְּהֵמָה טְמֵאָה בְּתַנּוּר אֶחָד, וְאַף עַל פִּי שֶׁאֵין נוֹגְעִים זֶה בָּזֶה. וְאִם צְלָאָן — הֲרֵי זֶה מֻתָּר, וַאֲפִלּוּ הָיְתָה הָאֲסוּרָה שְׁמֵנָה הַרְבֵּה וְהַמֻּתֶּרֶת רָזָה.
One does not roast kosher meat with the meat of נבילה or of an impure animal in one oven, even though they do not touch each other. But if one roasted them, it is permitted [בדיעבד], even if the forbidden one was very fatty and the permitted one lean.
Shulchan Aruch, Yoreh De'ah 108:1LEVEL 01
Hebrew text of the 7 se'ifim with a fluent English translation. ריחא לאו מילתא, the large open oven and one covered, a דבר חריף, the same pan (מחבת), the baker's peel (מרדה), hot bread on a wine barrel, and smelling a prohibition, explained with practical cases.
📖 StudyLEVEL 02
In-depth pilpul: the sugya of Pesachim 76 (ריחא מילתא?), Rashi's פיטום (the smell fattens), the yesod of דבר חריף, the distinction צלי / מחבת / בישול, the מחלוקת on a prohibition that forbids במשהו (chametz on Pesach), the צירוף of the 60 (Shach sk13, siman 111), חקירות and נפקא מינות.
📖 StudyLEVEL 03
Comparative tables (לכתחילה / בדיעבד, large-open or small-sealed oven, uncovered / covered, צלי / מחבת, חריף or not), golden rules, classic pitfalls (פיטום, ריחא במשהו, מרדה בת יומא, hot bread on a barrel) and memorization of the 7 se'ifim.
📖 StudyLEVEL 04
The practical halacha according to the Shach, Taz, Pri Megadim and Pitchei Teshuva, then the Sephardic poskim (Yabia Omer, Yalkut Yosef, Or LeTzion) and Ashkenazi poskim — baking meat and a dairy dish in one oven, microwave, smelling a non-kosher fragrance. Note: the Shulchan Aruch HaRav does not deal with this siman — this is a level of psak, not "Daat HaRav".
📖 StudyThe principle of the Gemara (Pesachim 76b, Avodah Zarah 66) is ריחא לאו מילתא: smell alone does not forbid. Therefore, according to the Shulchan Aruch (YD 108:1), if one inadvertently roasted permitted food and a prohibition in one oven, it is permitted בדיעבד — even if the prohibition was very fatty and the permitted lean. But לכתחילה one does not do so, unless the oven is large (12 עשרונים) and its mouth open, or one of the two is covered, and they do not touch each other. For practical halacha, consult your Rav.
The Rama notes several cases where ריחא מילתא, the smell forbidding even after the fact: (a) a דבר חריף (sharp food, and all the more so if it is the permitted item that is חריף), as long as both are uncovered; (b) prohibition and permitted item under one pan (מחבת) uncovered, treated like an actual cooking (cf. siman 93); (c) according to some, a prohibition that forbids במשהו such as chametz on Pesach. If one is covered (even with mere dough), or in case of loss, one relies on the lenient opinions. For practical halacha, consult your Rav.
Hot bread placed opposite the bung (מגופה) of an open barrel of יין נסך is forbidden, since it draws in the smell of the wine (se'if 4); but cold bread, or a sealed barrel, are permitted — except barley bread that is hot, forbidden even with a sealed barrel. As for smelling: one may draw in through the mouth the smell of the wine to test it, and smell a sack of pepper placed in a jar of wine; but it is forbidden to taste (Rivash), to sprinkle (לזלף) wine forbidden in benefit, and to smell the besamim of Havdala or those of ע״ז, כלאי הכרם and ערלה (se'ifim 5-7). For practical halacha, consult your Rav.