Half fill a medium-size bowl with ice, add the onion slices, cover with more ice, and fill bowl with cold water. Refrigerate about 30 minutes or up to a couple of hours.
Shortly before serving, fan the oranges on a large plate. Drain the onions, pat dry, and tuck the onion rings here and there among the orange slices. Lightly dust the salad with salt and pepper.
Drizzle the salad with the orange juice, scatter the olives over it, and sprinkle with about 2 tbsp. of olive oil. Taste for seasoning.