Preheat your oven to 375°F (190°C) and lightly grease a 9×13 inch baking dish.
Bring a large pot of salted water to a boil. Cook the ziti for 2 minutes less than the package instructions for al dente; drain and set aside.
Brown the ground beef in a large skillet over medium-high heat.
Add the onion to the skillet and sauté for 5 minutes until translucent.
Stir in the minced garlic and cook for 1 minute until fragrant.
Pour the marinara sauce into the skillet with the beef, then add the oregano, salt, and pepper. Simmer for 5 minutes.
In a medium bowl, stir together the ricotta and half of the parmesan cheese.
Combine the cooked pasta, beef sauce, and the ricotta mixture in a large bowl and toss gently.
Transfer the mixture to the prepared baking dish and top evenly with the shredded mozzarella and remaining parmesan.
Bake for 20 minutes, or until the cheese is bubbly and golden brown. Let sit for 5 minutes before serving.