Preheat oven to 375° F. Grease a 9x13-inch baking dish and set aside.
Bring a large pot of salted water to a boil. Once boiling, stir in the rigatoni and cook for the minimum cook time listed on the package. Then, drain (don't rinse) and set aside.
Meanwhile, heat the olive oil in a Dutch oven (or large skillet) over medium-high heat. Add the onion and cook until softened, 2-3 minutes, stirring occasionally.
Add the ground beef, and cook, crumbling, until beef is browned. Drain off excess grease as needed. Add the minced garlic and cook, stirring, for 30 seconds.
Add the marinara sauce, Italian seasoning and black pepper to the skillet and stir. Bring to a simmer and then reduce the heat to low. Cook at a low simmer for 10 minutes, stirring occasionally.
Remove the pan from the heat and stir in ¼ cup of grated Parmesan. Then gently stir in the cooked rigatoni. Transfer to the greased 9x13-inch baking dish.
Sprinkle the shredded mozzarella over the top of the pasta, followed by the remaining ¼ cup grated Parmesan. Sprinkle with the dried oregano.
Cover the baking dish with a piece of foil that has been sprayed with cooking spray on the underside (to prevent the cheese from sticking to the foil).
Bake, covered with foil, for 25 minutes. Then uncover and continue baking for 5-10 more minutes, until the rigatoni is hot and bubbly around the edges and lightly browned on top. Let rest for 5 minutes before serving. Serve with chopped fresh parsley, if desired.