Preheat the oven to 500 degrees Farenheit (yes, really!) Line a quarter-sheet pan or a 9x13 pan with parchment paper, then lightly grease it. Set aside.
In a large mixing bowl, whisk oats, rye and all-purpose flours, baking powder, baking soda, salt, and sugar, until fully combined.
Add the butter in chunks and use a pastry cutter or two forks to cut the butter into the dry ingredients until the mixture is a coarse texture and the butter is mostly incorporated.
Add the buttermilk to the mixing bowl and stir- a dough whisk or a spatula will work well. This is a very wet, sticky dough!
Scrape the dough into the prepared pan. Use wet hands to gently spread it out to evenly fill the pan.
Bake for 15-17 minutes, until lightly browned and springy to the touch. Allow to cool for 10 minutes in the pan, then cut into squares and serve warm.