Heat oil in a large pot over medium heat. Sauté onion until tender and translucent. Stir in the curry powder. Add the chopped carrots and stir until the carrots are well coated. Pour in the vegetable stock, and simmer until the carrots are soft, about 20 minutes.
Transfer soup to a blender, and purée until smooth (or use a hand held blender). Pour back into the pot, and thin with water to your preferred consistency.