In a large slow cooker, add brisket, carrots, celery, onion, potatoes, and mushrooms. Pour beef broth and water on top. Sprinkle thyme, black pepper, oregano, and salt over the top, and lightly mix with a fork (just to incorporate spices).
Cover and cook on low for 10 hours, or high for 6-7 hours. When beef is cooked through, it should easily shred. Using 2 forks, shred beef in pot, and then cover and cook for about 10-20 more minutes on high.
Serve and add salt and pepper to taste (if desired).
Notes
This recipe also freezes well. Add 3 cups to freezer ziplock bags, seal, and freeze. Can store for up for 6-12 months.