Preheat your oven to 350°F (175°C) and prepare a 9x9-inch baking pan by greasing it or lining it with parchment paper.

In a large mixing bowl, combine the rolled oats, all-purpose flour, brown sugar, granulated sugar, baking soda, and salt. Mix well. Then add the melted butter, lemon juice, lemon zest, and vanilla extract. Stir until combined and crumbly.

Reserve 1 cup of the oat mixture for the topping. Press the remaining mixture into the bottom of the prepared baking pan. Spread the fresh blueberries evenly over the pressed oat mixture, then sprinkle the reserved oat mixture on top.

Bake in the preheated oven for 25-30 minutes until golden brown and slightly crisp around the edges. Let cool in the pan for at least 15 minutes before slicing.