Peel the oranges and cut off the pith. Slice oranges into cubes removing any pips.
Remove and discard the outer leaves of radicchio. Thinly slice the remaining leaves and place in a medium bowl.
Add the orange slices. Drizzle with oil and balsamic vinegar and toss to coat. Season with salt, sugar(if desired) and freshly ground black pepper. Mix thoroughly to combine all ingredients.