Heat olive oil in a large skillet over medium heat. Add onions and sauté until starting to brown (about 10 minutes). Add the garlic and cook for one more minute.
Add the chicken, paprika, chili powder, cumin, oregano, cayenne, and salt. Turn the heat to high and use a wooden spoon or spatula to stir and break up the chicken into small chunks until the chicken is partially cooked (about 5 minutes). Stir in tomato sauce, reduce heat to low and simmer, stirring occasionally for about 10-15 minutes or until the chicken is cooked through. Sprinkle with fresh cilantro.
On individual plates pile some Doritos and top with some of the taco and a layer of shredded cheese. Microwave until the cheese has melted and top with ranch dressing, lettuce, tomato, and guacamole. You can also place the Doritos onto a baking sheet and top them with the taco meat and cheese and then bake at 350 degrees F. until the cheese has melted.