In a large resealable plastic bag, combine orange juice concentrate, 2 T. oil, soy sauce, and garlic. Add chicken; seal bag and turn to coat. Refrigerate for 2-3 hours. Remove chicken; reserve marinade.
In a skillet, cook chicken in remaining oil until juices run clear.
Meanwhile, in a saucepan sauté walnuts in butter until lightly browned; remove and set aside. Set aside 1/4 cup of green onions for garnish. Add remaining onions to saucepan; sauté until tender. Add reserved marinade and the next 8 ingredients. Bring to a rolling boil; boil for 2 minutes. Reduce heat; simmer, uncovered for 5-10 minutes or until sauce reaches desired consistency.
Serve chicken over rice; top with sauce and reserved walnuts and onions.