Bring 1 1/2 cups water to a boil in a medium saucepan; stir in couscous and oregano. Remove from heat; cover and let stand 5 minutes. Fluff with a fork. Combine couscous, tomatoes, and next 5 ingredients (tomatoes through chickpeas) in a bowl; set aside.
Combine 1/4 cup water and remaining ingredients; stir well with a whisk. Pour dressing over couscous mixture, tossing gently to coat.