In a large mixing bowl, combine the ground beef, breadcrumbs, grated Parmesan, chopped parsley, minced garlic, dried oregano, egg, salt, and pepper. Mix gently but thoroughly without overmixing.
Shape the mixture into small, bite-sized meatballs (about 1 to 1.5 inches). Heat a skillet over medium heat with a little oil and brown the meatballs on all sides.
Bring a large pot of salted water to a boil. Add rigatoni pasta and cook until al dente, about 1-2 minutes less than package instructions. Drain and set aside.
Preheat oven to 375°F (190°C). Spread a thin layer of marinara sauce in a baking dish. Add cooked rigatoni, then scatter browned meatballs on top. Pour remaining marinara sauce over everything.
Sprinkle shredded mozzarella cheese evenly over the top. Bake uncovered for 20-25 minutes until cheese is melted, bubbly, and golden brown.
Remove from oven and let sit for a few minutes before serving. Garnish with extra parsley or Parmesan if desired.