Preheat the oven to 350ºF (175ºC).
In a large mixing bowl, combine the 12 oz cooked chopped chicken, softened 8 oz cream cheese, 1/4 cup sour cream, 1/4 cup ranch dressing, 3/4 cup BBQ sauce, and 1 cup of the shredded cheddar; stir until evenly mixed.
Transfer the mixture to a lightly greased 8×8 (or 9×9) baking dish and spread into an even layer.
Sprinkle the remaining shredded cheddar cheese over the top, then scatter the chopped green onion evenly.
Bake for 25–30 minutes, until the dip is bubbly and the cheese is melted and lightly browned.
Let cool for a few minutes before serving.