Place the chicken thighs, chicken broth, diced onion, minced garlic, lemon juice, oregano, salt, and pepper into the slow cooker.
Cover and cook on low for 6 hours until the chicken is very tender.
Remove the chicken to a cutting board and shred it with two forks. Return the shredded chicken to the slow cooker.
Stir in the sliced kalamata olives and let everything warm for 5 more minutes.
Ladle the soup into bowls and top each serving with crumbled feta cheese and chopped fresh parsley.