Preheat oven to 375° F.
Add ricotta cheese, 2 ½ cups mozzarella cheese, ¼ cup shredded Parmesan cheese, egg, minced garlic, and oregano to bowl of food processor fitted with steel blade. Process for 10 seconds, or until all ingredients are well combined. Set aside.
Cook shells in boiling water with 1 teaspoon added salt for 1 minute less than indicated on package directions. Drain; rinse with cold water and thoroughly drain.
Spread ⅓ of Marinara sauce on bottom of 9 x 13-inch baking pan.
Fill piping bag fitted with large star tip (1M or larger) with cheese mixture. Pipe cheese mixture into shells. Place shells, open side up close together in pan. Spread remaining Marinara sauce over shells. Sprinkle with remaining ½ cup mozzarella cheese, and ¼ cup shredded Parmesan cheese.
Cover baking dish with aluminum foil, and bake in preheated 375° F oven for 25 minutes. Remove foil, and bake an additional 10 minutes, or until bubbly. Let stand for 10 minutes before serving.
Yield: 6 servings.