Heat olive oil in 8-quart saucepot over high heat and cook mushrooms, stirring frequently, until golden, about 4 minutes. Stir in onion and carrots and cook, stirring occasionally, until crisp-tender, about 3 minutes. Reduce heat to medium-high and stir in garlic and tomato paste and cook, stirring occasionally, 3 minutes.
Add water, barley, Knorr® Homestyle Stock - Vegetable and oregano. Bring to a boil over high heat. Reduce heat to low and simmer until barley is cooked, about 30 minutes. Stir in parsley. Serve, if desired, with hearty crusty bread.