In a large bowl, dissolve yeast and 1/2 teaspoon sugar in warm water; let stand until bubbles form on surface.
Whisk together remaining 3 tablespoons sugar, salt, and 3 cups flour.
Stir oil into yeast mixture; pour into flour mixture and beat until smooth.
Stir in enough remaining flour, 1/2 cup at a time, to form a soft dough.
Turn onto a floured surface; knead until smooth and elastic, 8-10 minutes.
Place in a greased bowl, turning once to grease the top. Cover and let rise in a warm place until doubled, 1-1/2 to 2 hours. Punch dough down.
Turn onto a lightly floured surface; divide dough in half.
Shape each into a loaf. Place in 2 greased 9x5-in. loaf pans. Cover and let rise until doubled, 1 to 1-1/2 hours.
Bake at 190° C until golden brown and bread sounds hollow when tapped or has reached an internal temperature of 100° C, 30-35 minutes.
Remove from pans to wire racks to cool.