Grate the vegan butter into small pieces using a box grater and place it in the freezer until ready for use. See Recipe Notes.
Preheat the oven to 450º.
Combine the oat milk and lemon juice to make a vegan buttermilk. Chill the vegan buttermilk in the refrigerator until ready for use.
Combine the flour, baking powder, baking soda and salt in a large mixing bowl.
Add the butter to the dry ingredients and use your hands to combine the butter with the flour until the flour has the consistency of sand with some larger, small pebble-sized butter chunks. Work quickly to avoid melting the butter with your hands. See Recipe Notes.
Add the vegan buttermilk to the dry ingredients and stir to combine.
Knead the biscuit dough on a lightly floured surface. Keep the kneading to a minimum so as not to melt the butter in the dough.
Using your hands, pat the dough into a 1” thick slab. Then use a food ring or the top of a glass to cut the biscuits. Gather the unused dough and form it again to cut more biscuits.
Place the biscuits on a baking sheet lined with a silicon baking mat and lightly brush with the melted vegan butter.
Bake the biscuits on the center rack for 12-15 minutes. The biscuits will be golden brown when done.