Add the oat flour, cornstarch, and salt to a bowl and whisk to combine.
Cut the cold butter into small cubes and add it to the bowl. Use a pastry cutter or your fingertips to work the butter into the flour until it looks like coarse crumbs with some pea-sized pieces.
Add the cold water a little at a time, mixing until the dough holds together when pressed.
Form the dough into a disc, wrap, and refrigerate for at least 30 minutes.
Roll the chilled dough between two sheets of parchment paper, then transfer it to a pie dish and press it in gently. Trim and crimp the edges as needed.