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 9×5-in. loaf pans. Cover and let rise until doubled, 1 to 1-1/2 hours.
Bake at 375° until golden brown and bread sounds hollow when tapped or has reached an internal temperature of 200°, 30-35 minutes. Remove from pans to wire racks to cool.