Preheat oven to 375°.
Arrange eggplant slices in a single layer on a baking sheet coated with cooking spray. Coat slices with cooking spray; sprinkle with salt. Bake at 375° for 16 minutes, turning eggplant over after 8 minutes. Combine cheese, oregano, pepper, and garlic in a bowl.
Arrange half of eggplant slices in an 8-inch square baking dish coated with cooking spray. Arrange half of tomato slices over eggplant slices. Top with half of zucchini slices; sprinkle with half of cheese mixture. Repeat procedure with remaining eggplant slices, tomato slices, zucchini slices, and cheese mixture.
Bake, covered, at 375° for 1 hour. Uncover and bake an additional 10 minutes or until vegetables are tender and cheese is golden brown.