In a large saucepan melt margarine over medium heat. Add onion, garlic, celery, and carrot. Stir and cook over medium heat until onion is slightly translucent (carrot and celery will remain crunchy).
Stir in milk and bring mixture to a gentle boil.
Remove one cup milk from the mixture. Stir flour into the one cup of milk, continue stirring until the flour has dissolved and there are no lumps. Return the milk and flour mixture to the saucepan. Stir in the parsley, oregano, and salt and pepper to taste. Cook until mixture thickens. Stir in the shrimp and heat through.
