In a large pot, heat the olive oil over medium heat. Add the diced onions, carrots, and celery. Sauté for about 5 minutes or until the vegetables begin to soften.
Add the minced garlic and sauté for an additional minute until fragrant.
Season the vegetables with thyme, oregano, salt, and pepper. Stir to combine.
Next, add the chicken thighs to the pot and pour in the chicken broth. Bring to a boil, then reduce the heat to a simmer. Cover and cook for 20 minutes, until the chicken is cooked through.
Remove the chicken from the pot and place it on a cutting board. Once cooled slightly, shred the chicken using two forks.
While the chicken is cooling, add the orzo pasta to the soup and cook according to package instructions (typically about 8-10 minutes).
Return the shredded chicken to the pot and stir in the lemon zest and lemon juice. Adjust salt and pepper to taste.
Allow the soup to simmer for an additional 2-3 minutes for the flavors to meld together.