Preheat your oven to 375°F (190°C) and prepare an 8-inch square baking pan by lining it with parchment paper.

Combine old-fashioned oats, all-purpose flour, light brown sugar, baking soda, salt, and cinnamon in a large mixing bowl.

Pour in melted unsalted butter and stir in vanilla extract, mixing until a crumbly texture forms.

Press half of the crumbly oat mixture into the bottom of the prepared baking pan to form a firm base.

In a separate bowl, toss blueberries with granulated sugar and cornstarch until well-coated.

Spread the blueberry mixture evenly over the pressed oat base, then crumble remaining oat mixture over the top.

Bake for 35-40 minutes or until the top is golden brown and blueberries are bubbling.

Remove from the oven and let cool completely on a wire rack before slicing into squares.