Heat the olive oil in a large pot over medium heat.
Add the diced onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and cook for an additional minute until fragrant.
Add diced carrots and celery; cook for 3-4 minutes, stirring occasionally.
Incorporate the zucchini and continue cooking for another 2 minutes.
Pour in the diced tomatoes with their juice, followed by vegetable broth and dried oregano.
Bring the mixture to a boil, then reduce heat and let it simmer for 15 minutes.
Stir in the chopped spinach just before serving.
Season with salt and pepper to taste before serving.