Heat olive oil in a large pot over medium heat.
Add the diced onion and cook until translucent, about 5 minutes.
Stir in the minced garlic and cook for another minute.
Add sliced carrots and diced celery; sauté for about 5 minutes.
Pour in the vegetable broth and bring to a boil.
Add the diced tomatoes, tortellini, and oregano; return to a boil.
Reduce heat and simmer for about 5-7 minutes or until tortellini is cooked.
Stir in the chopped spinach or kale and season with salt and pepper.
Serve hot, garnished with grated Parmesan if desired.