In a large bowl, cover the beans with cold water by 2 inches. Soak for at least 6 hours or up to overnight.
Drain and rinse the beans. In a 6-quart slow cooker, stir the beans, onions, tomatoes, garlic, molasses, cumin, chili powder, oregano, and 3 cups water. Cover and cook on low until the beans are tender, about 8 hours.