Ovens-Heating/Returning food into a hot oven on Shabbos:
Fulfilling the six conditions only allows one to return a pot of food onto a stove top, electric plate or Blech. However, one may never return the food into a hot oven even if all the 6 conditions are fulfilled, and even if the food was in the oven before Shabbos and then removed. Nevertheless, those who are lenient to heat and return food inside of an oven do not need to be protested, so long as all six required Chazarah conditions are fulfilled.
Q&A on returning foods into an oven
Is it permitted to return food into our modern-day ovens if the Chazara conditions are fulfilled?
Practically: Those which are lenient should place on the floor of the oven an empty upside-down pot. On top of this pot, they are to return the food.
· According to all, the oven must be properly covered in its inside walls as was explained in “The Laws of Shehiyah”.
May one return his Cholent to the crock pot upon fulfilling all six conditions? Does a crock pot have the status of a stove or an oven in regard to Chazara?
Returning food into the crock-pot is similar to returning it to an oven. Thus, seemingly whether this is allowed or not is dependent on the two opinions explained above. Thus, here too according to some it would be forbidden to return the cholent into the Crockpot even if all the other Chazara conditions have been fulfilled. However, there is room to say that by a crock pot it is permitted completely according to all.
 Admur 253:16“All the above [cases mentioned of which it is allowed to return the pot to the Kirah only apply] when he returns the pot onto the opening of the Kirah on its top, meaning that the walls of the pot protrude a little bit above the walls of the Kirah. However to return it into the oven itself, even with regards to our ovens [which open from the side and not from their top, and thus by our ovens the only place to return it is to inside the oven] is forbidden in all cases even if it was fueled with straw or twigs and [its coals] were swept or covered and the pot is still in ones hand and one had intention to return it.” and final ruling in 253:19 to be stringent; Michaber 253:2
Other opinions in Admur: The custom of those times was to be lenient to return the pot into our modern-day ovens, which have the same laws as returning a pot into the inside of a Kirah. [Admur 253:19; Rama 253:2 in name of Yeish Omrim; Ran; Kol Bo] The basis behind the lenient custom is because there are opinions which say that the Sages did not require one [upon removing the pot] to have intention to return it and for it to be kept it in his hand [until he returns it]. This is with exception to the case of when one removes the pot before Shabbos and now wants to return it on Shabbos, in which case [the Sages said that] if one did not have intention to return it, or he [did but] had already removed it from his hand, then [it is forbidden for him to return it because] it is considered like he is initially placing it on the Kirah on Shabbos, as when Shabbos first began the pot was not on the Kirah at all. It is based on this opinion that the world relies on to be furthermore lenient to say that even when the Sages prohibited to return the pot to the inside of the oven, it was only prohibited when one removed the pot from before Shabbos and it was in his hand until after Shabbos had begun [in which case he may return it on top of the Kirah, if the other conditions are fulfilled, but not inside the Kirah]. [Admur 253:19] Practically, one need not protest against those who are accustomed to follow the above leniencies although every person should be stringent on himself to not return the pot to the inside of a Kirah or Tanur even if one removed it from there after Shabbos had begun, [and] even if he had intention to return it, and it is still in his hands. [Admur 253:19]
 The reason: As it appears like one is cooking on Shabbos. [M”B 253:58]
Regarding the reason for this prohibition see Hearos Ubiurim [Tzemach Tzedek] Vol. 6 p. 22 where it suggests that this is due to that it appears like cooking on Shabbos.
 Shabbos Kehalacha Vol. 1 page 383; Piskeiy Teshuvos 253:8
 The Aruch Hashulchan 253:17 rules that it is allowed, as only by ovens which one can also cook on its top, as the ovens of back then, was it prohibited to return the food into them on Shabbos. Based on this ruling the Sheivet Haleivi 3:48 and Igros Moshe 4:74-26 rule that even ovens which contain a stove top attached to them, since they are two separate entities of heat, there is no difference between the oven and the stove. It is thus permitted to return food to inside the oven on Shabbos just as it is permitted to return onto the stove. [However to do so one must make sure before Shabbos to cover the walls of the oven in the way explained in the beginning of the chapter in order so it have a status of Ketumah.]
 Minchas Yitzchak 3:28 argues that it remains prohibited to return food to inside any oven, even of today. So plainly rules Shemiras Shabbos Kehilchasa 1:17, and so seems to be the opinion of Rav Shlomo Zalman Aurbach.
 As this makes the scenario more lenient according to some Poskim.
 Shabbos Kehalacha Vol. 1 page 387