Preheat the oven to 350F and line a 9*13 inch pan with parchment paper or lightly grease the pan.
Add the oat milk and apple cider vinegar to a large mixing bowl. Whisk together and let sit to curdle for 2-3 minutes to create the "vegan buttermilk".
Next, add the pumpkin puree, sugar, brown sugar, olive oil, and vanilla to the bowl of vegan buttermilk and mix all the wet ingredients together.
In a separate bowl, sift together the all-purpose flour, pumpkin pie spice, salt, baking powder, and baking soda. Once sifted, gradually add the dry ingredients to the mixing bowl of wet ingredients and whisk together until a smooth batter forms. I like to start with a whisk and switch to a spatula so I do not overmix the batter.
Pour the batter into the 9*13 inch pan and bake for 28-32 minutes or until the cake is cooked through.
Remove the cake from the oven and let it cool. If you used parchment paper you can transfer it to a wire rack to cool or let it cool in the baking dish. Once the cake has cooled, make the frosting.