Cut off top and bottom ends of oranges and stand up on one flat side. Using a paring knife, start at top of fruit and trim off all peel and white pith, following curve of fruit. Working over a bowl, slice between white membranes, releasing segments and allowing juice to drip into bowl. Squeeze any remaining juice into bowl. Cut orange segments into ½-inch cubes, removing any seeds. Add orange pieces, fennel, olive oil, salt and pepper into bowl with juice; toss gently to combine. Garnish with 2 tablespoons chopped fennel fronds and toss again. Let sit to allow flavors to meld for at least 30 minutes.