Preheat oven to 350°F. Spray a 3-quart baking dish with cooking spray and set aside.
Boil mostaccioli noodles to al dente according to package directions, then drain and rinse under cool water. Return the pasta to the empty pot.
In a large skillet set over medium-high heat, brown the Italian sausage, onion, and garlic until cooked through.
Add the marinara sauce and stir, simmering for 3-4 minutes.
Pour the meat sauce into the pot of pasta and mix well.
In a large bowl, combine the egg, ricotta cheese, oregano, 1 cup mozzarella cheese, and ¼ cup Parmesan cheese.
Place half of the pasta mixture into the prepared baking dish.
Spread the ricotta mixture evenly across the pasta, then top with the remaining pasta mixture. Sprinkle the remaining cheeses on top.
Cover with aluminum foil and bake for 30 minutes.
Remove the aluminum foil and bake an additional 10 minutes, until the cheese is melted. Garnish with grated Parmesan cheese, chopped parsley, and serve.
