DO-AHEAD TIP: Chill salad for up to 2 hours, if desired.
In a large bowl, combine chicken, raisins, walnuts and green onions; blend well.
In a small bowl, combine mayonnaise, sour cream, lemon juice, oregano and salt and pepper to taste; pour over chicken mixture and toss gently.
Cover and refrigerate for up to 2 hours, if desired. Serve over 2 cups of mixed baby greens.