In a large pot or Dutch oven, heat the olive oil over medium heat. Add the Italian sausage and cook, breaking it up with a spoon, until browned. Remove the sausage with a slotted spoon and set aside, leaving the drippings in the pot.
Add the chopped onion, carrots, and celery to the pot. Cook, stirring occasionally, until softened, about 5-7 minutes.
Stir in the minced garlic and cook for 1 minute more until fragrant.
Pour in the chicken broth and add the diced tomatoes (with their juice), oregano, basil, and red pepper flakes (if using). Bring the soup to a simmer.
Add the cooked Italian sausage back to the pot along with the ditalini pasta. Stir well.
Continue to simmer for 10-12 minutes, or until the ditalini is tender and cooked through. Season with salt and black pepper to taste.
Ladle the soup into bowls. Garnish with grated Parmesan cheese and fresh parsley, if desired. Serve hot.
Notes
This soup is even better the next day as the flavors meld together. Feel free to add other vegetables like spinach or kale in the last few minutes of cooking.