Whisk oil, vinegar, orange juice, mustard, shallot, orange zest and pepper. Reserve 1/4 cup dressing in measuring cup.
Bring 6 cups water to a boil in large sauce pan. Add green beans and salt. Cook uncovered, until green beans are slightly tender but still crisp, about 4 minutes.
Using slotted spoon, transfer beans to plate lined with paper towels to drain briefly, then add to bowl with dressing. Toss to coat.
Add potatoes to still simmering water. Bring back to a boil and cook until paring knife can be inserted into potatoes with no resistance, 7 to 10 minutes. Using slotted spoons transfer potatoes to colander and drain briefly. Add hot potatoes to bowl with beans and dressing.
Add parsley, chives and tarragon and reserved dressing to bowl. Toss gently to combine and serve.