Preheat oven to 375°F (190°C).
Cook spaghetti in salted boiling water until al dente according to package directions; drain thoroughly.
Brown the Italian sausage in a large skillet over medium-high heat until no pink remains; drain excess rendered fat.
In a large mixing bowl, incorporate the cooked spaghetti, browned sausage, and marinara sauce until evenly coated.
Transfer 50 percent of the spaghetti mixture into a 9x13 inch baking dish.
Layer 1 cup of mozzarella and half of the pepperoni, peppers, and onions over the first layer of pasta.
Add the remaining spaghetti mixture to the dish as a second layer.
Top with the remaining mozzarella, pepperoni, peppers, onions, black olives, Parmesan cheese, and dried oregano.
Bake for 25 to 30 minutes until the cheese is fully melted and the edges are bubbling.