Chop the chicken into bite-sized pieces. Season with salt and pepper.
Heat the oil in a dutch oven or large pot over medium heat. Add the chicken and cook, stirring often, until golden on all sides, about 5 minutes. Remove chicken to a bowl and set aside.
Add the onions, carrots, and celery to the pot and cook over medium heat, stirring often, until softened, about 5 minutes. Stir in the garlic and cook for 30 seconds more.
Add the chicken stock, bay leaf, and oregano to the pot and bring to a boil.
Add the orzo and chicken to the pot and simmer for 10 minutes or until the orzo has softened.
Stir in the spinach and cook until wilted, about 2 minutes.
Remove from the heat and stir in the lemon juice. Season with salt and pepper, as needed.