Using a paring knife, cut away peel and white pith from 2 medium blood oranges or navel oranges; discard. Cut between membranes to release segments into a medium bowl; discard membranes.
Add 1 large fennel bulb, halved lengthwise, cored, very thinly sliced crosswise, 1 medium red onion, very thinly sliced, 16 mint leaves, torn, and 3 Tbsp. extra-virgin olive oil to bowl; toss to combine. Season liberally with kosher salt and freshly ground pepper.
Transfer salad to a platter. Scatter 20 oil-cured black olives over.
