To make the vinaigrette, combine the orange juice, lime juice, vinegar, mustard, shallot, salt, and pepper in a small bowl or jar. Let stand for 10 minutes.
Slice off the top and bottom ends of the oranges and place the flat bottom on a cutting board. With a sharp chef ’s knife, cut off the peel in strips all the way around the orange, following the curve of the fruit. Slice the oranges into ¼-inch-thick (6 mm) rounds.
Arrange the watermelon and orange over a platter, then drizzle with the vinaigrette. Just before serving, garnish with the basil, almonds, and feta cheese, if using, and drizzle with olive oil.
