Preheat the oven: Start by preheating your oven to 375°F (190°C). Spray an 8-inch casserole dish with nonstick spray to prevent sticking.
Cook the pasta: In a large pot, bring water to a boil. Add 2 cups uncooked penne pasta and cook according to the package instructions until al dente. Drain the pasta and drizzle it with olive oil to prevent sticking.
Prepare the sauce mixture: In a mixing bowl, combine 8 oz softened cream cheese, ¾ cup ranch dressing, and ⅓ cup hot sauce. Mix until creamy and smooth, ensuring all ingredients are well blended.
Add chicken and pasta: Stir in 2 cups shredded cooked chicken and ½ cup shredded mozzarella into the cream cheese mixture. Gently fold in the pasta until evenly coated in the sauce.
Transfer to the baking dish: Spoon the pasta and chicken mixture into the prepared casserole dish, spreading it evenly.
Top with cheese: Sprinkle the remaining 1 cup mozzarella and ½ cup shredded cheddar evenly over the pasta mixture.
Bake: Place the casserole in the preheated oven and bake for 20 minutes, or until the cheese is melted and bubbly, and the top is lightly golden brown.
Garnish and serve: After baking, remove from the oven and let it cool for a few minutes. Serve with sliced green onions, and offer extra hot sauce and ranch dressing on the side for those who love added flavor.