Menachos, Chapter Five, Mishnah 9 - the final mishnah of the chapter. This mishnah deals with the two voluntary meal offerings that are baked in an oven.
The two types of meal offering baked in an oven:
Challos - something like a relatively thick matzah, similar to Sephardic matzah.
Rekikin - thin and brittle, similar to Ashkenazic matzah.
Someone who takes upon himself to bring a meal offering made in an oven may bring either of these two types. But once he has specifically used the word "oven," he is obligated to have the meal offering baked in an oven and nowhere else.
What is an oven:
Chazal have different names for the various baking vessels. The oven is built something like a cone: wide at the bottom, where the fire burns, and narrow at the top, like a furnace. Its heat is intense, and the bread bakes on its inner walls. Other vessels are the kupach, which is smaller than an oven, and the kirah, which is wider and gives off less heat.
What does not count as "the work of an oven":
Ma'afeh kupach - the kupach is like a small oven, but its name is not "oven."
Ma'afei re'afim - re'afim are earthenware tiles. The bread has to be baked on the wall of the oven, not on a scorching clay tile, and therefore this is not the work of an oven.
Ma'afei yoros ha'Arviyim - a yorah is a sort of pot, and that is not an oven. In practice this refers to a hole dug in the ground and plastered with clay, with the fire burning inside it and the bread baking on its walls - very similar to the way an oven bakes, and even so it is not an oven but a different vessel.
Therefore, someone who obligated himself by saying "I take upon myself a meal offering baked in an oven" and then prepared his offering in one of these has not fulfilled his obligation, since his commitment was to bring it baked in an oven.
Rabbi Yehudah says: if he wishes, he may bring one baked in a kupach. In his view, even though the kupach does not carry the name "oven," it is essentially a small oven that works in the very same way, and that is enough. The halachah, however, does not follow Rabbi Yehudah; an oven is required, one that people actually call an oven.
Half challos and half rekikin:
One who says "I take upon myself a baked meal offering" has two options open to him: challos or rekikin. A voluntary meal offering is divided into ten loaves from the issaron, and this raises the question: since the phrase "baked meal offering" includes both challos and rekikin, may he split it and bring five challos and five rekikin?
The Mishnah states: "Harei alai minchas ma'afeh - lo yavi mechtzah challos u'mechtzah rekikin" - one who says, "It is upon me to bring a baked meal offering," may not bring half loaves and half wafers. According to the first Tanna, he may not bring half the quantity as wafers and half as loaves; rather, all ten must be of whichever type he chooses, since the entire issaron must form a single unit prepared in one manner.
Rabbi Shimon matir, shehu korban echad - Rabbi Shimon permits it, because it is one offering. In his view, since the two halves, five and another five, are both part of a single issaron, and all of it is baked in an oven, there is no disqualification.
In summary: the halachah does not follow Rabbi Shimon. One who obligates himself to bring a meal offering baked in an oven may choose either loaves or wafers, but he must bring all ten of one type only.