Heat the oil in a large skillet over medium-high heat and brown the roast on all sides. Transfer the roast to a 4- to 6-quart slow cooker. To the fat remaining in the skillet, add the onions, celery, carrots, and garlic. Cook, stirring frequently, until tender, about 10 minutes. Add the tomato paste and stir to coat the vegetables; transfer to the cooker. Pour the wine into the skillet and scrape up any browned bits; add the contents of the skillet to the cooker, along with the mushrooms, salt, oregano, and tomatoes (plus 1 cup of their liquid). Cook 8 hours on low heat, or 4 hours on high heat.