Heat the olive oil in a cast-iron pot over medium-high heat. Add the chicken thighs and brown them for 2 minutes per side.
Add the ancho chili powder, cumin, water and salt.
Bring the liquid to a simmer. Cover and cook over low heat for about 1 hour, or until the chicken is tender enough to shred and has reached at least 165°F (74°C).
Shred the chicken with 2 forks, then return it to the cooking liquid.
If using soft tortillas, warm them in the microwave for about 30 seconds, or until soft.
Divide the chicken between the tortillas or taco shells. Add your favorite toppings and serve with salsa, guacamole and/or sour cream.