Preheat your oven to 375°F.
Cook the manicotti shells according to package instructions until al dente, then drain and set aside to cool slightly.
In a mixing bowl, combine ricotta cheese, 1/2 cup mozzarella cheese, Parmesan cheese, egg, oregano, garlic powder, salt, and black pepper. Mix until well combined.
Using a spoon or piping bag, carefully fill each manicotti shell with the cheese mixture.
Spread 1 cup of marinara sauce on the bottom of a 9x13-inch baking dish. Arrange the filled manicotti shells in a single layer over the sauce.
Pour the remaining marinara sauce over the top of the shells.
Sprinkle the remaining mozzarella cheese over the top of the sauce. Cover the dish with aluminum foil.
Bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let the manicotti cool for 5 minutes before serving. Garnish with fresh basil if desired.