Heat olive oil in a skillet over medium-high heat. Add onions, turkey, oregano, salt, garlic powder, and a few grinds of pepper. Saute, stirring occasionally and crumbling meat as it cooks. While meat cooks, chop your other ingredients to save prep time. Divide chopped lettuce evenly between four large salad bowls. When turkey is cooked through, stir in ketchup. Drain meat well in a colander, and allow to cool slightly. Scoop meat over lettuce, and top each salad with tomatoes, pickles, and two tablespoons of cheese. Drizzle salad dressing over the top to taste.