Preheat the oven to 375 degrees and grease a 9x13 baking dish. Cook the penne in a large pot of salted water until al dente, then drain and set aside.

Heat the olive oil in a large skillet over medium heat. Add the Italian sausage and cook until browned, breaking it up with a spoon as it cooks.

Add the sliced onion, bell peppers, and minced garlic to the skillet. Cook for 5 minutes until the vegetables soften. Stir in the dried oregano, salt, and pepper.

Pour the marinara sauce into the skillet and stir to combine with the sausage and vegetables. Add the drained pasta and mix until everything is evenly coated.

Transfer the mixture to the prepared baking dish. Sprinkle the mozzarella and Parmesan evenly over the top.

Bake for 20 minutes until the cheese is melted and bubbly. Let the casserole rest for a few minutes before serving.