Whisk flour, rolled oat and sugar in a mixing bowl.Grate the cold butter into dry ingredients using the large holes of box grater.
Using /pastry cutter/ mixer or your finger tips, quickly rub in the butter until the mixture resembles bread crumbs.
Gradually add a tablespoon of ice water at a time, use your fingers to press the crumbs firmly together to form a ball of dough.Wrap the dough in plastic, chill for 30 minutes-1 hour.
For filling, combine sliced peaches, sugar and flour..
Roll out 2/3 of the pastry on floured board to fit 9 x 13 in jelly roll pan with some overhang. Carefully transfer onto baking sheet, pressing into the corners. Fold dough over to create thicker edge. Heat the oven @ 180 C.
Spoon the filling onto prepared dough.. Crumble the balance of the pastry over the filling.
Bake until topping is crisp for 35-45 minutes. Cut into slices to serve.