Heat olive oil in a large pot over medium heat. Add sausage and brown for 3 minutes, breaking it up into bite-size pieces with a wooden spoon.
Add onion, garlic, oregano, salt, pepper, and red pepper flakes. Cook until onion is softened and sausage is browned, about 7 more minutes.
Stir in tomato paste and cook for 2 minutes more.
Add diced tomatoes, bay leaves, and chicken stock. Bring just to a boil, then reduce heat and simmer for 15 minutes.
To serve, add cooked pasta to individual bowls and ladle soup over the top. Dollop the top of the soup with generous spoonfuls of ricotta cheese and a few tablespoons of chunk mozzarella. Place under the broiler for a minute to warm and slightly brown cheese.
Finish with fresh basil and an additional sprinkle of salt and pepper.