Bring a large pot of salted water to a boil and cook the jumbo pasta shells according to package directions until al dente; drain and rinse with cold water to stop cooking, then set aside to cool slightly.
Preheat the oven to 350°F (175°C). Grease a 9 x 13 inch baking dish and spread a thin layer (about 1/2 cup) of marinara sauce evenly over the bottom.
In a large mixing bowl, combine the ricotta, shredded mozzarella, 3/4 cup grated Parmesan, eggs, dried oregano, salt, black pepper, and the squeezed-dry spinach; mix until evenly incorporated.
Fill each cooled pasta shell with about 2 tablespoons of the spinach-ricotta mixture and arrange the filled shells seam-side up in the prepared baking dish.
Pour the remaining marinara sauce over the arranged shells, spreading gently to cover, and sprinkle additional grated Parmesan over the top if desired.
Cover the dish tightly with aluminum foil and bake in the preheated oven for about 35 minutes, until the filling is heated through and the sauce is bubbly.
Remove foil and let rest a few minutes before serving.