Preheat your oven to 375°F (190°C). Line an 8x8" baking pan with parchment paper or give it a light grease.
To make the oatmeal roll dough, add the wet ingredients to a large mixing bowl, including 2-1/4 cups of plain Greek yogurt, 1 tablespoon of apple cider vinegar, and 2 tablespoons of honey. Whisk until it’s smooth and creamy.
Add the dry ingredients, including 1-1/2 cups oat flour, 1/2 cup gluten-free flour, 1 tablespoon baking powder, 1 teaspoon baking soda, and 1/4 teaspoon salt. Stir and press with a sturdy spatula, or the dough hook in a stand mixer, until the ingredients are incorporated. All the dry ingredients should be incorporated but the dough should still be tacky when touched.
Divide the dough evenly into 9 pieces. Pour a small amount of oil into your hands and roll each into a smooth ball. Then, place the dough balls in your baking pan in 3 rows of 3.
Separate the egg yolk from an egg white and brush egg white onto the tops of the rolls with a pastry brush. Sprinkle a few oats on top of the muffins.
Bake for 25-28 minutes or until the tops are golden brown and firm to the touch. Allow the rolls to cool before slicing.