Peel the oranges and use a small sharp knife to remove the white pith from the entire outside of the orange. Cut the orange crosswise into 1/4-inch slices, then cut each of the slices into 4 quarters. Remove any seeds, taking care to leave the pieces intact and in a nice shape.

In a small container or bowl combine the vinegar, olive oil, shallots, mustard, and salt and pepper.

In a large serving bowl combine the arugula and endive and olives if using. Add the dressing and toss to combine. Add the orange pieces and gently toss again to combine.