Bring a large pot of salted water to a boil.

Cook the manicotti shells until al dente according to package instructions.

In a mixing bowl, combine ricotta cheese, 1/2 cup mozzarella, Parmesan, egg, oregano, garlic powder, salt, and pepper.

Preheat the oven to 375°F (190°C).

Using a spoon or piping bag, fill each manicotti shell with the ricotta mixture.

Spread a layer of marinara sauce on the bottom of the baking dish.

Carefully place the filled manicotti in the baking dish, ensuring they are not crowded.

Pour the remaining marinara sauce over the manicotti and sprinkle with the remaining mozzarella.

Bake in the preheated oven for 30 minutes, covered with foil.

Allow the manicotti to sit for 5-10 minutes before serving.