Preheat your oven to 350°F and line a baking sheet with parchment paper.
In a small bowl, mix ground flaxseed and water to create a flax egg, then set aside for 5 minutes.
In a large mixing bowl, cream together the vegan shortening and brown sugar until fluffy.
Add the vanilla extract and the flax egg mixture to the bowl and stir to combine.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Stir in the rolled oats to finish the vegan oatmeal cookies dough.
Roll the dough into 1-inch balls and place on the prepared baking sheet, spacing 2 inches apart.
Flatten each cookie ball slightly with a flat-bottomed glass.
Bake for 10–12 minutes, or until edges are golden but centers are soft.
Let cookies cool on the baking sheet for 10–15 minutes before transferring to a wire rack.