In a large skillet, heat the oil and butter. Once melted, add the onions and saute on medium low heat until caramelized, about 30 minutes. Set aside and cool completely.
In a medium bowl, mix together the sour cream, mayonnaise, garlic powder, dried minced onion, Worcestershire, and pepper.
Fold in the caramelized onions.
Chill for at least 30 minutes (overnight is better), before serving.