In 10-inch nonstick skillet, heat oil over medium heat. Add chicken; cook, stirring occasionally, until brown on both sides. Sprinkle with taco seasoning mix. Cook, stirring occasionally, just until chicken is no longer pink in center.
While chicken is cooking, heat shells and assemble toppings. Divide chicken mixture among shells; add shredded cheese, cheese 'n salsa dip, and additional desired fresh toppings.