Preheat the oven to 375°F and lightly grease an 8×8-inch baking dish. Set it aside while you prepare the topping and filling.
In a medium mixing bowl, combine the flour, oats, brown sugar, baking powder, cinnamon, and salt. Add the softened vegan butter and mix with a pastry blender, fork, or your hands until the mixture is crumbly and well combined. Place the bowl in the refrigerator to chill while you make the filling.
In a large mixing bowl, stir together the vegan butter and flour until smooth. Add the lemon juice, oat milk, vanilla extract, brown sugar, cinnamon, and salt. Stir until blended, then add the sliced apples and toss to coat completely.
Spread the apple mixture evenly into the prepared baking dish. Sprinkle the chilled streusel topping evenly over the apples.
Transfer the dish to the oven and bake, uncovered, for 30 minutes, or until the top is golden brown and the filling is bubbling around the edges.
Let the crisp cool for at least 10 minutes before serving to allow the filling to set. Serve warm as-is or with a drizzle of vegan caramel sauce.