Heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook until softened, about 5 minutes.
Add the minced garlic, chili powder, cumin, oregano, salt, and pepper to the skillet. Cook for 1 minute until fragrant.
Place the chicken breasts in the skillet. Pour in the chicken broth.
Bring the liquid to a simmer, then reduce the heat to low, cover the skillet, and cook for 15 to 20 minutes, or until the chicken is cooked through (internal temperature reaches 165°F).
Remove the chicken breasts from the skillet and place them on a cutting board. Shred the chicken using two forks.
Return the shredded chicken to the skillet with the remaining liquid. Stir to coat the chicken completely. Cook uncovered for 2-3 minutes to allow the sauce to thicken slightly.
Warm the tortillas according to package directions (in a dry skillet or microwave).
Serve the shredded chicken filling in the warm tortillas. Top with your preferred toppings.
Notes
For a slow cooker method, place all ingredients except tortillas and toppings in the slow cooker. Cook on low for 4 hours or high for 2 hours. Shred the chicken in the cooker and stir to combine before serving.
Use pre-made taco seasoning instead of the individual spices for a faster preparation.
For juicier chicken, do not overcook the meat in the skillet.