Preheat oven to 300°.
Heat sugar, orange juice and orange zest until sugar dissolves, about 3 minutes; reserve syrup.
Peel oranges and remove all pith. Separate orange segments and reserve. Thoroughly mix dry ingredients; reserve.
Whisk together remaining liquid ingredients.
Stir liquid mixture into dry ingredients until just blended. Pour into a buttered 6-cup round or square cake pan. Bake until golden brown and firm to the touch, about 30 minutes.
Reheat orange syrup. A bit at a time, spoon syrup over warm cake, letting syrup absorb into cake before adding more. Cool cake on a rack. To serve, garnish each piece with some of the reserved orange segments.