In a medium bowl, add the sliced red onion. Top with vinegar and a sprinkle of sea salt. Set aside for the time it takes to prepare the rest of the dish, or until the onions have softened. Drain.
In the meantime, cut off just the tips of the oranges and use a sharp paring knife to remove the skin and the white pith. Thinly slice the oranges and remove any pits.
Arrange the oranges on a serving plate. Top with the marinated onions, sprinkle the olives on top, and finish with a drizzle of extra virgin olive oil. Add a pinch of flaky sea salt (keeping in mind that the olives are salty too, so don't overdo it), and fresh ground pepper.
Notes
Feel free to use any variety of orange you prefer. Growing up, my mother and grandmother used only sweet black olives. I personally like the salty tang of the oil cured olives against the sweet citrus, but feel free to make this suited to your flavor profile.