In a medium bowl, combine the flour, oats, brown sugar, granulated sugar, cinnamon, and salt. Mix well. Add the butter and use a fork or pastry blender to cut it into the flour mixture. The mixture should be crumbly with some large pieces. Try not to overmix. Set aside.
¾ cup all-purpose flour, 1 cup old-fashioned oats, ½ cup brown sugar, ¼ cup granulated sugar, 1 teaspoon ground cinnamon, ⅛ teaspoon kosher salt, 10 tablespoons unsalted butter