In a large pot over medium heat, add the olive oil and Italian sausage. Cook until browned, breaking it apart with a spoon, about 5-7 minutes. Drain excess fat if necessary.
Add the diced onion, garlic, carrots, and celery to the pot. Sauté for about 5 minutes, until the vegetables are softened.
Stir in the oregano, basil, and red pepper flakes (if using). Cook for an additional minute until fragrant.
Pour in the chicken broth and diced tomatoes. Bring the mixture to a boil.
Add the ditalini pasta and cook according to package instructions, usually about 8-10 minutes, until al dente.
Reduce the heat to low and stir in the heavy cream and Parmesan cheese. Mix until the cheese is melted and the soup is creamy.
Season with salt and pepper to taste. Serve hot, garnished with fresh parsley.