Sort and wash beans; place in a large saucepan. Cover with water to 2 inches above beans; cover and let stand 8 hours. Drain beans. Add 1 quart water and return beans to pan. Bring to a boil over medium heat. Reduce heat, and simmer, partially covered, for 30 minutes. Cool. Drain beans in a colander over a bowl, reserving cooking liquid; set beans aside. Add enough water to cooking liquid to equal 3 cups; set aside.
Heat the oil in a Dutch oven over medium-high heat. Add leeks, oregano, 1 teaspoon salt, and garlic; sauté 5 minutes or until leeks are tender. Reduce heat to low; cover and cook 10 minutes, stirring occasionally. Transfer leek mixture to a large bowl; add greens. Toss well to combine.
Place half of greens mixture in bottom of Dutch oven; top with beans. Spread remaining greens mixture over beans. Sprinkle with 1/2 teaspoon salt and pepper. Pour reserved liquid over greens. Cover and simmer over medium heat 1 hour or until beans are tender, stirring once. Uncover and stir in tomatoes; simmer 10 minutes.