Heat the olive oil in a large Dutch oven over medium heat. Stir in the carrots, celery, and onions,¼ teaspoon of salt, and black pepper to taste, and cook until just softened, 3-5 minutes. Stir in the mushrooms, and cook until all the vegetables are softened, 5-6 more minutes. Add the garlic and oregano and cook until fragrant, about 30 seconds.