Zest one of the oranges; set aside. Slice off the ends of each orange. Working with one orange at a time, set an orange on end; using a small knife, slice off and discard the peel and the white pith. Holding the orange in your hand, cut the orange segments away from their membranes. Place segments in a bowl and set aside. Over a separate bowl, squeeze the remaining orange pulp to extract any remaining juice. Repeat with the remaining orange. Whisk 4 tablespoons oil, lemon juice, oregano, and scallions into the orange juice, and season with salt and pepper. Set the dressing aside.