Mix the flour, rolled oats, sugar, cinnamon, and salt well in a medium bowl. Then add the cooled melted butter and stir with a spoon or spatula to mix together. Will resemble crumbles, but will stick together.
Place ⅔ of the mixture into the prepared baking pan. Spread and press the mixture evenly using your hands or the bottom of a measuring cup.
Next, cover the crust with the peach mixture, spooning out the peaches by leaving most of the juice in the bowl as much as possible. Over excess juice is not needed, so it's better to drain the juice off.
Sprinkle the remaining dough over the peaches. Use your fingertips to press to the crumbles down a little so they can stick to peaches.
Bake for 35-37 minutes until the top becomes golden brown and the crumbles are crunchy.
Let cool at room temperature for almost 1 hour.
Keep bars in the fridge for extra 1,5 hours until it is completely cool so you can have nicely cut bars.