Heat oil in a saucepan over medium heat. Add onion, carrots, garlic and ginger; cook until vegetables are slightly softened, about 5 min. Stir in flour and curry powder. While stirring, slowly pour in broth and orange juice; bring to a simmer. Cook until carrots are tender, about 20 min.
Using a blender or food processor, purée soup; return to pan. Bring to a simmer; add orange zest and salt. Pour into bowls; garnish with some of the yogourt and parsley (if using) before serving.