Place the beans in a large saucepan and cover with water; bring to a boil. Cook, uncovered, for 8-10 minutes or until crisp-tender. Drain and rinse in cold water.
In a large bowl, combine the beans, oranges and onion. In a small bowl, whisk the oil, vinegar, sugar, salt and pepper. Pour over bean mixture and toss to coat. Cover and refrigerate for at least 2 hours. Just before serving, add the salad greens, avocado and pecans; toss gently.