Heat oil in a large soup pot over medium heat. Add onion, carrots, celery and garlic; cook and stir until onion has softened and turned translucent, about 5 minutes. Stir in mushrooms and continue to cook for about 3 minutes.

Pour in chicken broth, add barley, and bring to a boil. Reduce heat to low, cover, and simmer until barley is tender, about 40 to 50 minutes.

Season with salt and pepper before serving.