Preparation

• In a small bowl, combine the sugar, yeast, and 1/4 cup warm (90°F) water. Let stand until the yeast dissolves and the mixture is foamy, about 15 minutes.
• Meanwhile, use a rasp-style grater to remove 1-1/2 Tbs. zest from the oranges. In a small bowl, toss the zest with the salt.
• In a large bowl, combine 3-3/4 cups of the flour with the salt and zest, using your fingers to make sure the zest is well distributed with no clumps. Make a well in the center. Add the yeast mixture, 2 Tbs. of the oil, and 1 cup warm water, and stir with a wooden spoon until a rough dough forms.
• Oil a large, clean mixing bowl with about 1 Tbs. of the oil.
• Turn the dough onto a lightly floured work surface and knead in the remaining 1/4 cup flour. Continue kneading until the dough is soft, smooth, and elastic, about 5 minutes. Shape the dough into a ball, and transfer it to the prepared bowl. Turn the dough over in the bowl to thoroughly coat with oil. Cover the bowl with plastic wrap and a kitchen towel, and let rise until doubled in size, about 1 hour.
• Meanwhile, peel and slice the oranges, and then cut each slice into 4 to 6 pieces.
• Position a rack in the center of the oven, and heat to 400°F.
• Roll the dough out on a piece of parchment sized for a large baking sheet until it is almost the size of the parchment. Transfer on the parchment to a large rimmed baking sheet, fitting it to fill. Using wet fingertips, make indentations across the surface. Brush all over with 2 Tbs. oil, letting the oil pool in the indentations.
• Scatter the orange pieces and olives over the surface of the dough, and gently press them into the dough. Spread the onion slices evenly over the dough and gently press them in. Sprinkle all over with flaky salt, a little black pepper, and the oregano, if you like. Bake until risen and golden on top and bottom, 25 to 35 minutes. (Areas around the orange pieces will remain light.)
• Slide a metal spatula under the focaccia and transfer it to a rack. Let cool briefly before cutting and serving.