Place the chicken breasts in the bottom of the slow cooker. Add the diced onion, minced garlic, and the can of diced tomatoes with green chilies.
Pour the chicken broth over the ingredients. Sprinkle in the chili powder, cumin, oregano, salt, and black pepper.
Cover and cook on low for 6 to 7 hours until the chicken is tender and easy to shred.
Remove the chicken from the slow cooker and shred it with two forks. Return the shredded chicken to the broth and stir to combine.
Ladle the soup into bowls. Top each serving with diced avocado and chopped cilantro. Serve with lime wedges on the side.