Heat the oven to 375°F, grease a 9-by-13-inch baking dish and boil the rigatoni 2 minutes short of tender.
Fold the drained pasta with the cooked chicken, marinara, mozzarella, 1/2 cup Parmesan and oregano.
Spread the mixture in the baking dish and cover it loosely with foil.
Bake for 18 minutes while you mix the panko, olive oil, garlic powder and remaining Parmesan.
Remove the foil, scatter the crumbs on top and bake until browned and bubbling, 10 to 12 minutes.
Rest the casserole for 10 minutes before serving.