Cook the jumbo pasta shells according to the package instructions until al dente. Drain and set aside.
In a medium bowl, mix together the ricotta cheese, half of the mozzarella cheese, chopped spinach, minced garlic, dried oregano, salt, and pepper.
Stuff each pasta shell with the cheese and spinach mixture using a spoon.
In a large skillet, heat olive oil over medium heat. Add the marinara sauce and bring to a simmer.
Gently place the stuffed shells into the sauce in the skillet.
Sprinkle the remaining mozzarella cheese over the top of the shells.
Cover the skillet with a lid and let it simmer for about 20 minutes, or until the cheese is melted and the sauce is bubbly.
Remove the skillet from heat and let it sit for a few minutes before serving.
Garnish with grated Parmesan cheese and fresh basil before serving.