Combine soy sauce, ketchup, cola, brown sugar, garlic, ginger, salt, and black pepper in a bowl to make the marinade.
Place the chicken thighs in a resealable plastic bag and pour the marinade over them.
Seal the bag and refrigerate for at least 1 hour or overnight.
Preheat the oven to 375°F (190°C).
Place the marinated chicken thighs in a baking dish and pour the marinade over them.
Bake for 35-40 minutes or until the chicken is cooked through and the sauce is thickened.
Garnish with chopped green onions before serving.