Preheat your oven to 375°F (190°C).
Cook the pasta shells according to package instructions until al dente. Drain and set aside.
In a large skillet, brown the ground beef over medium heat. Drain excess fat.
Add minced garlic, oregano, salt, and pepper to the beef. Cook for 2-3 minutes.
In a bowl, combine the cooked beef mixture with ricotta and half of the mozzarella cheese.
Stuff each pasta shell with the beef and cheese mixture.
Spread a layer of marinara sauce in a baking dish.
Place the stuffed shells in the dish, seam side up.
Pour the remaining marinara sauce over the shells.
Sprinkle the remaining mozzarella and Parmesan cheese on top.
Cover with foil and bake for 20 minutes, then remove the foil and bake for an additional 10 minutes until bubbly.