Combine the oats, flour, sugar, baking soda and salt in a large bowl.
Cut the butter into small pieces and pinch it into the bowl of dry ingredients. You can use your fingers or pastry cutters to mix it into the ingredients until the mixture looks like coarse crumbs. Add the chopped chocolate into the mix and stir to combine.
Drizzle the buttermilk into the bowl and gather the mixture together with your hands until it forms a dough.
Turn the dough out onto the countertop and flatten it into a disk. Lightly dust the counter with flour and roll the dough out until it is ¼-inch thick. Use a 3-inch round cutter to cut out the cookies. Transfer the cut out cookies to a parchment lined baking sheet. Gather up the dough scraps, re-roll it and cut out more cookies until all the dough has been used up.
Place the cut out cookies into the freezer while you pre-heat the oven to 400˚F.
Bake the cookies at 400˚F for 10 minutes. You can go a minute or two longer if you like the cookies very crispy, but keep your eyes on them.