Preheat oven to 350 F. Cook pasta according to package directions, drain. Place pasta into a large bowl, mix with marinara sauce.
Spread pasta out into a large greased baking dish. Sprinkle shredded mozzarella cheese on top of the casserole.
Place pepperoni slices on top of pasta. Sprinkle with dried oregano. Bake for 20-25 minutes.