In a medium bowl, combine softened butter, oats, flour, and brown sugar. Use your hands to break the butter up into pea-size bits. The mixture is ready when it resembles soft snow - when you squeeze the mixture, it forms into a 'snowball'. 

In a large mixing bowl, combine sugar, cornstarch, and peaches. Mix well. Pour into a large baking dish and top with oat mixture. 

Bake at 350 F for 40 minutes.