Witch's Oven's sacrifice is a cost. You cannot respond to the paying of costs, they are paid for as part of activating the ability. By the time the controller of the Oven gets priority (they get it first, because they had priority prior the activation of the Oven) the Familiar is already in the graveyard. And only when they pass priority are you getting priority for the first time to respond.

Essentially, as long as they keep the Oven untapped, you cannot exile the Familiar. If they activate it first, the Familiar is in the graveyard before you receive priority to respond. If you try to remove the Familiar first, they can respond by sacrificing it to the Oven. Likewise, if the Familiar is in the graveyard, you cannot successfully exile it from there as long as they have a Food to sacrifice to bring it back in response.