Dissolve sugar in 1/2 cup warm water in a large mixing bowl which has first been rinsed with hot water. Sprinkle yeast on top and let stand for 10 minutes. Stir to dissolve.

Combine with oil, warm water, sugar, salt, eggs and half of the flour. Beat well. Stir in remaining flour. Dough should be sticky. Cover dough and let rest for 10 minutes. Turn out onto a floured board and knead for 10 minutes, adding flour as needed.

Round up in a greased bowl. Cover and let rise in a warm place until double in bulk, about 1 1/2 to 2 hours. Punch down, cover and let rise again until double, about 45 minutes.

Divide dough into 3 equal parts. (If you want to make 2 smaller challahs, first divide dough in half, then divide each half into 3 equal parts.) Shape into strands. Place on a lightly greased baking sheet and braid loosely. Fasten ends securely. Cover with a damp cloth and let rise until double. Brush with beaten egg yolk and sprinkle with seeds. Bake at 400°F for 30 minutes, until golden brown. (If making 2 smaller challahs, baking time will be about 25 minutes.)