Preheat the oven to 350°F (175°C).

Grease or line an 8x8-inch (20x20 cm) baking pan with parchment paper, allowing some overhang for easy removal.

In a large bowl, combine 2 cups of oats, 1 ¾ cups all-purpose flour, ¾ cup packed brown sugar, ½ teaspoon cinnamon, ½ teaspoon baking soda, and ½ teaspoon salt. Stir until well combined.

Add the melted butter to the mixture, and stir until the ingredients are evenly moistened and form a crumbly texture.

Press ⅔ of the oat mixture into the bottom of the prepared pan. Use your fingers or the back of a spoon to evenly flatten and pack the mixture into a firm crust.

Bake the crust in the preheated oven for about 10 minutes until it begins to firm up and lightly brown. Remove from the oven and let it cool for a few minutes.

Spread the blueberry jam evenly over the pre-baked oat crust. Ensure you cover the entire surface with a smooth layer.

Sprinkle fresh or frozen blueberries over the jam layer. Distribute them evenly for a balanced blueberry flavor in each bite.

Crumble the remaining oat mixture over the blueberry layer. This will create a textured and crispy topping that complements the soft blueberry filling.

Gently press the crumble layer down with your hands or a spatula to ensure it adheres to the filling.

Return the pan to the oven and bake for another 30-35 minutes, or until the top is golden brown and the blueberry filling is bubbling around the edges.

Remove from the oven and allow the bars to cool in the pan for at least 20-30 minutes before slicing.