Combine first 4 ingredients; sprinkle over both sides of chicken. Place chicken on rack of a broiler pan coated with cooking spray. Broil 5 1/2 inches from heat 12 minutes or until done, turning once.
Meanwhile, combine broth and next 4 ingredients in a saucepan; bring to a boil. Remove from heat, and stir in couscous. Cover and let stand 5 minutes or until liquid is absorbed and couscous is tender.
Spoon couscous evenly onto 4 serving plates; top each serving with a chicken breast half. If desired, garnish with lemon slices and oregano sprigs.