Preheat your oven to 375°F (190°C).
Cook the ziti according to package instructions until al dente, then drain.
In a skillet, brown the Italian sausage over medium heat, breaking it apart with a spatula.
Add minced garlic and dried oregano to the skillet and cook for an additional minute until fragrant.
In a bowl, mix ricotta cheese, egg, salt, pepper, and half of the Parmesan cheese until well-combined.
Combine the drained pasta, marinara sauce, and the sausage mixture in a large bowl.
Layer half of the pasta mixture in a greased baking dish, then dollop half of the ricotta mixture on top.
Add the remaining pasta then the remaining ricotta mixture, smoothing it out evenly.
Top with shredded mozzarella and the rest of the Parmesan cheese.
Cover with foil and bake for 25 minutes, then remove the foil and bake for an additional 15 minutes.
Let the baked ziti cool for about 10 minutes before serving.