Juice one of the oranges and measure out ¼ cup of juice. Whisk the juice together with the olive oil, vinegar, salt, and pepper. (Dressing can be made ahead. Store, covered, in the refrigerator until ready to serve. Whisk before pouring over the salad.)

Cut a slice off the end of each of the remaining oranges and cut off the skin and white pith. Slice the oranges into ¼-inch thick rounds then cut each round in quarters. (Oranges can be prepared ahead. Store, covered, in the refrigerator.) Place the orange slices in a serving bowl and add the arugula and dressing. Toss to coat. Add the pomegranate seeds, toss, and serve.