Drain beans and cook them, covered, in a large pot of boiling water until tender, about 1 1/2 hours. Drain.
Heat oil in a soup pot over medium heat. Sauté onions, celery and carrots for 3 to 5 minutes. Add water, the cooked beans, tomatoes, oregano, salt, cayenne and pepper. Simmer vegetables are tender, about 30 minutes. Taste and adjust seasonings.