Preheat oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.
Boil a large pot of salted water and cook penne until 2 minutes before reaching al dente consistency; drain and set aside.
In a large skillet over medium-high heat, brown the Italian sausage, breaking it into small crumbles until cooked through.
Add the diced onion to the skillet with the sausage and sauté for 5 minutes until translucent.
Incorporate the minced garlic and cook for 60 seconds until fragrant.
Stir in the marinara sauce, oregano, and red pepper flakes; reduce heat and simmer for 5 minutes.
In a large mixing bowl, fold together the par-cooked penne, the sausage sauce mixture, and the ricotta cheese until thoroughly combined.
Transfer the pasta mixture into the prepared baking dish and spread into an even layer.
Top the dish with an even distribution of shredded mozzarella and grated parmesan cheese.
Bake for 20 to 25 minutes until the cheese is melted, golden-brown, and bubbling.
Remove from oven and garnish with fresh parsley before resting for 5 minutes prior to service.