Heat the oven to 425°F, toss the zucchini with 1 teaspoon salt and let it drain in a colander for 15 minutes.
Blot the zucchini dry, toss it with the olive oil and roast it on a sheet pan for 10 minutes.
Reduce the oven to 375°F and combine the rotisserie chicken, marinara, oregano, black pepper and remaining salt.
Layer half the zucchini, half the chicken sauce and half the mozzarella in a 2 1/2-quart baking dish and repeat.
Sprinkle the Parmesan over the top and bake until bubbling and browned, 20 to 25 minutes.
Rest the casserole for 10 minutes before serving.