1 jar (16 ounces) store-bought marinara sauce (2 cups)
Preheat oven to 450°. Arrange eggplant slices in a single layer on two rimmed baking sheets. Brush lightly on both sides with oil; season with salt and pepper. Roast until eggplant is tender and golden, turning halfway through, 25 to 30 minutes.
Meanwhile, in a medium bowl, whisk together ricotta, eggs, ½ cup Parmesan, oregano, 1 ½ teaspoons salt, and ¼ teaspoons pepper. Brush an 8-inch square baking dish with oil. Lay a fourth of eggplant slices in bottom of prepared dish; spread with half of marinara sauce. Top with another fourth of eggplant; spread with half of ricotta mixture. Repeat layers, ending with ricotta; sprinkle with remaining ½ cup Parmesan. Bake until bubbling and golden, 20 to 25 minutes. Cool 10 minutes before serving.