Pour boiling water over oats and shortening in a bowl. Stir until shortening melts; let stand for 20 minutes, giving it a stir once in a while.
Meanwhile, dissolve sugar in lukewarm water. Add yeast and let stand for 10 minutes. Stir briskly with a fork.
Stir yeast mixture into the partially cooled oat mixture with molasses and salt. Let cool to lukewarm, 5 to 10 minutes. Beat in 2 1/2 cups of flour, gradually adding remaining flour as needed.
Turn dough onto a floured surface and knead, 8 to 10 minutes. Shape into a smooth ball and place in a greased bowl, rolling the dough so the top is greased. Cover and let double in size, about 1 1/2 hours.
Punch dough down and shape into 4 loaves. Place into greased 8 1/2x4 1/2-inch bread pans. Cover and let rise until doubled in size, about 1 hour.
Preheat the oven to 400 degrees F (200 degrees C).
Bake in preheated oven until tops are golden brown, 30 to 35 minutes.