Trim the fennel, discarding tough outer layers, then halve the bulb and thinly slice it crosswise. Thinly slice the red onion and set both aside.
Slice a thin disk from the top and bottom of each orange. Stand an orange upright and cut downward to remove peel and white pith. Slice crosswise into 0.5-inch rounds, repeating with all oranges and saving any juices.
Arrange the orange rounds slightly overlapping on a large platter and spoon any collected juices over them.
Scatter the sliced fennel and red onion evenly over the oranges, then add the pitted black olives.
Drizzle extra-virgin olive oil over the salad and season with kosher salt and freshly ground black pepper to taste. Serve right away or let stand 5–10 minutes to meld flavors.