In a large mixing bowl, combine the active sourdough starter and warm water. Stir gently to combine.
Add the all-purpose flour, granulated sugar, and salt to the bowl. Mix with a wooden spoon or dough whisk until a shaggy dough forms.
Add the olive oil and knead the dough in the bowl or on a lightly floured surface for about 8-10 minutes until the dough is smooth and elastic. The dough will be slightly sticky but manageable.
Lightly oil a clean bowl and place the dough inside, turning once to coat with oil. Cover the bowl with a clean kitchen towel or plastic wrap.
Let the dough rise at room temperature (around 70-75°F/21-24°C) for 6 to 8 hours, or until it has doubled in size and is bubbly.
After the first rise, gently deflate the dough and turn it out onto a lightly floured surface. Shape it into a tight loaf by folding the dough edges under to create surface tension.
Place the shaped dough seam-side down into a greased 9×5-inch (23×13 cm) loaf pan. Cover loosely with a towel and let it rise for another 1 to 1.5 hours, until it has risen about 1 inch above the rim of the pan.
Preheat your oven to 375°F (190°C) about 20 minutes before baking.
Bake the bread for 35 to 40 minutes, or until the crust is golden brown and the loaf sounds hollow when tapped on the bottom.
Remove the bread from the oven and let it cool in the pan for 10 minutes. Then transfer it to a wire rack to cool completely before slicing.