Season the chicken with salt and pepper and place in a 6- to 7-qt. slow cooker. Top with the garlic and oregano. Add the tomatoes and broth. Cover and cook on low heat until the chicken is tender and an instant-read thermometer inserted into a thigh away from the bone registers 165 degrees , about 5 hours.
Top with the olives and feta, cover and cook for 15 minutes.