Heat oil in skillet over medium heat. Cook and stir onion in hot oil until translucent, about 5 minutes. Break ground turkey into small chunks and add to skillet; cook and stir until beginning to brown, 2 to 3 minutes. Sprinkle taco seasoning over the turkey mixture; continue to cook and stir until turkey is completely browned through, 5 to 7 minutes more.
Pile lettuce into the bottom of a large salad bowl; top with the turkey mixture, corn, black beans, avocado, green onions, Cheddar cheese, and spicy ranch dressing.