Preheat your oven to 350°F (175°C).
In a large pot of salted boiling water, cook the cheese tortellini according to package instructions until al dente.
While the tortellini cooks, mix marinara sauce, olive oil, Italian seasoning, salt, and pepper in a large bowl.
Once cooked, drain the tortellini and add it to the mixing bowl with the marinara sauce. Stir gently to combine.
Add half of the mozzarella and half of the cheddar cheese to the mixture, and fold them in gently.
Transfer the mixture to a greased baking dish, spreading it evenly.
Top with the remaining mozzarella and cheddar cheese, and scatter the pepperoni slices on top.
Cover with aluminum foil and bake for 20 minutes. Then, uncover and bake for an additional 10-15 minutes until cheese is bubbly and golden brown.
Let the casserole cool for a few minutes before serving.