Heat oil in a large skillet and brown the sausage and sauté the onions for about 3 minutes.
Add the bell peppers, garlic,red pepper flakes, and oregano and saute for about 4 minutes.
Stir in the tomatoes, tomato paste, water,and heavy cream and simmer until sauce start to thicken, about 5 minutes.
Stir in the dried Ziti, and stir well to coat the pasta.
Transfer to slow cooker, cover, and cook on high for 1 hour 30 minutes hours. Check the slow cooker after 1 hour to ensure the noodles don't overcook, you may need to cook a little less or a little more time depending on your slow cooker.
Sprinkle with mozzarelly and serve.
Notes
if doubling the recipe, use a larger slow cooker.
your favorite jarred sauce can be used instead of the tomatoes and spices.