Heat olive oil in a large pot or Dutch oven over medium-high heat. Add Italian sausage, breaking it up with a wooden spoon. Cook until browned and no longer pink, about 5-7 minutes. Drain any excess grease and set the cooked sausage aside.

Reduce heat to medium. Add chopped onion, carrots, celery, and red bell pepper to the pot. Sauté until vegetables are softened, about 8-10 minutes. Add minced garlic and cook for another minute until fragrant.

Stir in the crushed tomatoes, chicken broth, water, dried oregano, dried basil, red pepper flakes (if using), salt, and black pepper. Bring the mixture to a simmer, then reduce heat to low, cover, and let it cook for 10-15 minutes to allow the flavors to meld.

Stir the cooked sausage back into the pot. Add the orzo pasta. Increase heat to medium-low and continue to simmer, stirring occasionally, until the orzo is al dente, about 8-10 minutes (or according to package directions). The soup will thicken as the orzo cooks. If the soup becomes too thick, add a little more water or broth.

Stir in the baby spinach and cook just until it wilts, about 1-2 minutes.

Taste and adjust seasonings as needed. Ladle the hot soup into bowls. Garnish generously with grated Parmesan cheese and fresh chopped parsley before serving.