Trim the zucchini and cut it in half crosswise. Cut each half lengthwise into 4 slices.
Arrange alternating slices of the zucchini and tomato in a 12-by-9-by-2-inch gratin dish and pour the olive oil on top. (The gratin can be prepared to this point up to a day ahead, covered, and refrigerated.)
FOR THE TOPPING: Combine the cheese, bread, oregano, salt, and pepper in a mini-chop and process to crumbs. Transfer the mixture to a bowl and stir in the oil. Set aside.
At serving time, sprinkle the gratin with the topping. Bake for 35 minutes, or until the vegetables are tender and moist and the top is nicely brown.