in a very large bowl, whisk together 2 cups of the whole wheat flour and white bread flour, rolled oats, flaxseeds, brown suagr, yeast and salt. add water and butter, beat until smooth, occasionally scraping down the sides of the bowl, about 3 minutes. with wooden spoon, stir in oat bran and 2 cups of the remaining whole wheat flour to make soft sticky dough.
turn out onto lightly floured surface. knead for 10 minutes, adding enough of the remaining flour to make dough smooth and elastic. place in greased bowl, turning to grease all over. cover with plastic wrap, let rise in warm, draft-free place until double in size, 1 to 1 1/2 hours.
punch down dough, turn out onto lightly floured surface, knead into ball, divide in half and shape into ball again. gently pull each ball into 12x7-inch rectangle.
starting at narrow end, roll up into cylinder; pinch along seam to seal. fit seam side down, into two greased 9x5-inch loaf pans. cover with plastic wrap and let rise in warm, draft free place until doubled in size, about 1 hour.
bake in centre of 375*F oven until golden brown and load sounds hollow when tapped on bottom, 30 to 35 minutes.