Cut peels and white pith from oranges. Working over a bowl, cut the segments from surrounding membranes. Squeeze the peels and membranes over the bowl to extract about 3 tablespoons juice before discarding them. Transfer the segments with a slotted spoon to a small bowl. Whisk oil, vinegar, shallot, sugar and salt into the juice. Return the segments to the bowl along with the snow peas and jicama; toss with the dressing.