In a medium skillet, heat the olive oil over medium heat. Add the diced onion and cook for about 2 minutes, until softened. Stir in the minced garlic and cook for 1 more minute, just until fragrant.
Transfer the onion and garlic mixture to the crock pot. Add the chicken breasts, chicken broth, celery, carrots, fresh thyme, and bay leaf. Cover and cook on high for 2½ to 3 hours, or until the chicken is cooked through and the vegetables are tender.
Remove the chicken from the crock pot and shred it using two forks.
Stir the orzo into the soup, cover, and cook for another 30 minutes, or until the pasta is tender.
Remove the bay leaf and thyme sprigs. Return the shredded chicken to the crock pot, stir in the lemon juice, and season with salt and pepper to taste. Serve warm.