Heat a large skillet over medium-high heat. Cook and stir Italian sausage in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease.
Arrange taco shells upright in a 9x13-inch baking dish. Sprinkle 2 tablespoons mozzarella cheese along the bottom of each taco shell.
Divide Italian sausage, pepperoni, onion, green bell pepper, and olives among the taco shells. Top each with 1 to 2 tablespoons pizza sauce, more mozzarella cheese, Parmesan cheese, dried oregano, and red pepper flakes.
Bake in the preheated oven until cheese is melted, about 10 minutes. Garnish with fresh oregano.