Preheat the oven to 350°F (175°C) and grease a 9x13 inch baking dish.
In a bowl, mix together the flour, rolled oats, brown sugar, baking soda, and salt. Add the softened butter and mix until crumbly, leaving some larger chunks for the topping.
Press about two-thirds of the crumb mixture into the bottom of the prepared baking dish to form a crust.
In another bowl, combine the sliced peaches, granulated sugar, lemon juice, cornstarch, and cinnamon. Toss to ensure the peaches are well-coated.
Spread the peach filling evenly over the crust in the baking dish.
Sprinkle the remaining crumb mixture evenly over the peach filling.
Bake in the preheated oven for about 30 minutes or until the top is golden brown and the filling is bubbly.
Allow to cool in the baking dish for at least 10 minutes before slicing into bars.