Heat olive oil in a Dutch oven over medium-high heat. Add onion, and sauté 3 minutes. Add oregano and garlic, and sauté 30 seconds. Add 5 3/4 cups Chicken Stock and potato. Bring to a boil; reduce heat, and simmer for 15 minutes or until the potato is tender. Mash potato in stock mixture. Add celery and carrot; bring to a boil, reduce heat, and simmer for 5 minutes. Add the chicken, spinach, and salt; bring to a boil, reduce heat, and simmer 3 minutes. Add noodles; bring to a boil, and cook 5 minutes.
Combine 1/4 cup stock and flour in a small bowl; stir well with a whisk. Stir flour mixture into soup; simmer 3 minutes. Stir in pepper.