Bring 1/2 cup water to a boil in a small saucepan; stir in couscous. Remove from heat, cover, and let stand 5 minutes. Fluff with a fork. Place couscous in a fine-mesh strainer; rinse with cold water. Drain.
Combine couscous, chicken, and next 6 ingredients (through oregano) in a medium bowl. Stir in oil and salt. Place 3 lettuce leaves on each of 4 plates; spoon about 1/4 cup chicken mixture in the center of each leaf.