Preheat the oven to 350°F (175°C).
Combine the oat milk and vinegar in a small bowl. Allow the mixture to stand for several minutes, giving it time to curdle slightly and develop the consistency of a dairy-free buttermilk.
In a large separate bowl, thoroughly whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and salt. Set the dry mixture aside.
Place the softened plant-based butter, granulated sugar, olive oil, and vanilla extract in another mixing bowl. Beat with an electric mixer for several minutes, continuing until the mixture becomes notably pale, creamy, light, and fluffy.
Add the prepared dry ingredients and the oat milk mixture to the creamed butter mixture in alternating portions. Work in approximately two to three batches, mixing well after each addition.
Continue mixing only until the batter is smooth, light, and evenly combined, with no visible lumps remaining.
Transfer the finished batter to an ungreased angel food cake pan, spreading it evenly as needed.
Bake for 35–45 minutes, or until the surface springs back when lightly pressed and a toothpick inserted into the center emerges clean.
Leave the baked cake in its pan to cool for 1 hour.
After the initial cooling period, carefully remove the cake from the pan and transfer it to a wire cooling rack. Allow it to cool completely before slicing and serving.
Once fully cooled, serve the cake as desired with vegan whipped cream and mixed berries.