Fill a large pot with salted water and bring it to a boil. Add the jumbo pasta shells and cook according to package directions until al dente, about 9 to 11 minutes. Drain and rinse briefly under cold water to stop cooking. Set aside to cool.
In a mixing bowl, combine ricotta cheese, shredded mozzarella, grated Parmesan, egg, dried oregano, garlic powder, salt, and black pepper. Mix well until smooth. Fold in the chopped fresh basil.
Preheat oven to 375°F (190°C). Spoon the cheese mixture into each cooled pasta shell carefully, then place the stuffed shells seam side up in a lightly greased baking dish.
Pour marinara sauce evenly over the shells and drizzle with olive oil.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove foil and bake an additional 10 minutes until bubbly and golden. Let rest for 5 minutes before serving.
