Using a paring knife, remove peel and white pith from the oranges. Slice crosswise into thin rounds, discarding any seeds. Place oranges on a serving platter. Top with the pecorino, olives, and as much onion as you'd like.
Cut the mandarin in half and squeeze juice into a small bowl. Drizzle in the oil (start with 1/4 cup and use more to taste), whisking until combined. Stir in the oregano. Season dressing with sea salt and freshly ground pepper or chili flakes.
Drizzle as much dressing as desired over the salad and let infuse for a few minutes before serving.