Stir the oat milk and lemon juice together in a cup. Let sit for a few minutes in the refrigerator to keep cold while continuing with the next steps.
Combine all the dry ingredients in bowl.
In a food processor or high-powered blender, pulse the compound butter with the dry ingredients until it is sandy in texture with a few pieces the size of peas. Transfer to a large bowl.
Pour the milk lemon mixture into the butter flour mixture. Stir with a spoon, until combined. If there are dry bits at the bottom of the bowl, use your hands to knead the dough together. The dough should be a bit on the dry side.
Pour the dough onto a lightly floured work area forming a cohesive dough mound. Roll out the dough with a floured rolling pin. The shape and thickness doesn't matter much at this point, but roll until about 1/2 inch thick.
Fold the dough like a letter, folding the top third toward the center and then the bottom third toward the center, on top of the first third. (see video)
Rotate dough 90 degrees. Then roll out the dough again and repeat the folding and rotating. Repeat 3-4 more times. As you repeat this process, you're dough should become rectangular.
After the final fold, roll the dough to a thickness of about 1-1.5 inches. Freeze the dough for about an hour.
With a sharp knife, cut the edges off the dough. This will allow for a more even rise and allow the layers to show. Cut the dough into desired shapes (6 square biscuits or 4-8 triangular biscuits).
If you're prepping ahead and plan to bake these later, wrap your dough and freeze. When you're ready to bake, continue with next step.
Brush the tops with melted butter and sprinkle with a bit of course salt. Bake at 425F for about 20-25 min. Rotate your pan midway if your biscuits aren't browning evenly.