Combine the chili powder, cumin, salt, pepper, paprika, garlic powder, oregano, and onion powder in a small dish and stir to combine into a taco spice mix. Place the cut up chicken in a zip-seal plastic bag and add the spice mix. Seal the bag and shake/massage the contents until the chicken pieces are coated with the spices.
Add the oil to a large saute pan/walled skillet and bring over med-high heat. Once the oil is hot, add the chicken pieces (you don’t want to overcrowd the pan, so depending on the size of your skillet you may want to work in two batches instead of one) and stir to break them up and coat them in the oil. Cook the chicken for 8-10 minutes, stirring every couple minutes. Once the chicken is cooked through, remove from heat and cover.
Bring a skillet to medium heat and mist each side of a tortilla with cooking spray. Place the tortilla in and pan and let it warm for about a minute. Flip and warm for another minute. Transfer to a plate and repeat with the remaining tortillas.
To build each taco, take a tortilla and fill it with around 1.75 ounces of cooked chicken. Top with lettuce and tomato and enjoy!