Place 3 3/4 cups (500 g) of the flour, salt, and zest in a large mixing bowl and rub together with your fingers until blended and there are no clumps of zest; make a well in the center of the flour. Pour 2 tablespoons oil, the yeast mixture, and remaining water into the well and stir with a wooden spoon until a rough dough forms; if there are any pockets of flour that won’t blend in, add 1–2 tablespoons more warm water at a time, only as needed.
Turn the dough out onto a lightly floured work surface and knead in the remaining 1/4 cup (40 g) flour. Knead the dough for 6 minutes, dusting both the dough and the work surface lightly with more flour to keep the dough from sticking. The dough should be soft, smooth, and elastic.
Oil a large, clean mixing bowl with 1 tablespoon of the oil. Place the ball of dough in the bowl, turning to coat the surface of the dough with oil. Cover the bowl with plastic wrap and a kitchen towel and let rise for 1 hour until double in size.
Prepare the toppings by peeling the oranges, cutting away all the white pith, and slicing across the core into 1/4-inch (1/2 cm) slices, about 6 slices per orange. If you prefer, slice each round into 4 triangles. Peel and trim the onion and slice as thinly as possible—cut the onion in half if easier—separating the slices into rings.
Preheat oven to 400 degrees F (200 degrees C).
Scrape the risen dough onto a lightly floured work surface and roll out into a 10 x 14-inch (25 x 35 cm) rectangle. Transfer to a parchment-lined baking sheet and roll and press back into shape. If you like, use wet fingertips to make indentations across the surface of the dough where a little oil can pool. Brush dough with remaining 1 tablespoon oil and arrange the oranges on the surface; pressing gently into the dough. Spread the onions evenly over the focaccia. Dot with the olives, pressing firmly into the dough, and dust with salt, pepper, and oregano. Bake for 30–40 minutes until risen and golden.