1) Preheat the oven to 500°F. This temperature isn't a misprint - this is a wet dough, and the high temperature works fine. Grease a 9" x 13" pan.
2) In a large bowl, whisk the oats and flours with the baking powder, baking soda, salt, and sugar.
3) Work the butter into the dry ingredients until thoroughly distributed. Stir in the buttermilk. (A dough whisk is the tool of choice here.) The mixture will be very sticky; that's OK.
4) Transfer the dough to the prepared baking sheet and, using wet hands, pat it out so it fills the pan.
5) Bake the bread for 15 to 17 minutes, until the top is light golden brown and springs back when gently touched. Remove the bread from the oven and cool it on a rack before slicing.