Heat the oil in a small pot over medium-low heat. Place the onions and garlic in the pot. When the garlic is browned, added the carrots and celery, and sweat out the vegetables for 3 to 4 minutes. Add the potatoes, and let it cook for another two minutes.
Add the chickpea broth, water, crushed tomatoes, and dried oregano. If you want the chickpeas to be softer when served, add them now. Otherwise, wait until the water boils before adding the chickpeas.
When the soup boils, stir in the fish sauce, salt and pepper. Continue cooking until the potatoes are fully cooked or easily pierced with a fork.
Serve with chopped fresh basil.