Preheat the oven to 375 degrees F and lightly grease a 9x13 inch baking dish.
Bring a large pot of salted water to a boil and cook the ziti until just al dente. Drain and set aside.
Heat the olive oil in a large skillet over medium heat. Add the Italian sausage and cook, breaking it up with a spoon, until browned.
Add the diced onion and minced garlic to the skillet. Cook for 3 to 4 minutes until the onion softens.
Stir in the fresh spinach and cook until wilted. Season with salt, pepper, and oregano.
Add the marinara sauce to the skillet and simmer for 2 minutes. Remove from heat.
In a large bowl, combine the cooked ziti, sausage mixture, ricotta cheese, and half of the mozzarella. Mix gently until evenly coated.
Transfer the mixture to the prepared baking dish. Sprinkle the remaining mozzarella and Parmesan evenly over the top.
Bake for 20 to 25 minutes until the cheese is melted and lightly golden. Let rest for 5 minutes before serving.