I swear "la cuptor" can't decide if it wants to be "in the oven" or "on the oven". Incredibly frustrating.
Why don't you need to add any definite article to "cuptor" in this case? (i.e. why is it not "Femeia are o varză la cuptorul"?)
AFAIK most prepositions (like la) don't take definite articles, so nouns following them will appear in their indefinite/non-suffixed forms