Preheat the oven to 350°F (175°C).
Cook the Rotini according to package instructions until al dente. Drain and set aside.
In a large mixing bowl, combine the cooked Rotini, pizza sauce, diced pepperoni, dried oregano, and 1 1/2 cups of shredded mozzarella cheese. Mix until well combined.
Transfer the mixture into a greased 9x13 inch baking dish. Spread it evenly across the dish.
Top with the remaining 1 cup of shredded mozzarella cheese and arrange the whole pepperoni on top.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it cool for a few minutes before serving.