Cook the pasta in plenty of boiling salted water until firm to the bite, according to package directions. Ladle out 1/2 cup of the cooking liquid; reserve. Remove from the heat and add 1 cup cold water to stop cooking; drain. In a large bowl, combine pasta, the reserved pasta-cooking water and 1 tablespoon of the olive oil; let stand until pasta is at room temperature, about 10 minutes.
Add the remaining 3 tablespoons olive oil, the oranges, celery, red onion, celery leaves, lemon juice, olives, salt and black pepper. Toss to blend.
Serve at room temperature or chill before serving.
If chilled, taste and correct seasoning, if necessary. Add walnuts, if using, just before serving.