1. Preheat the oven to 400°F and line a baking sheet with parchment paper.
2. In a small bowl or liquid measuring cup, combine the nondairy milk and apple cider vinegar.
3. In a large bowl, combine the oat flour, almond flour, tapioca flour, salt, and baking powder and mix well.
4. Add the butter to the dry ingredients, and cut it into the flour with a fork until small pieces form.
5. Add the wet ingredients to the dry ingredients and stir well. The dough will be sticky and a little wet.
6. Transfer the dough to a well-floured surface and sprinkle more flour on top of the dough (2 to 3 tablespoons, depending on how wet the dough is).
7. Form the dough into a disk that’s about 3 inches thick, and slice the disk into 8 pieces with a knife. Form each piece into a biscuit shape (round or square).
8. Transfer the biscuits to the prepared baking sheet, leaving about 1 inch between them, and brush the tops with avocado oil.
9. Bake until the biscuits are just golden, 18 to 20 minutes. Serve warm. Leftover biscuits will keep in an airtight container at room temperature for 2 days or in the freezer for up to 2 months. Reheat in the oven or microwave once thawed.