Preheat oven to 375°. Grease an 8 inch, or 7 x 11 inch baking dish.
Place the mixed berries in greased baking dish; sprinkle sugar, corn starch and cinnamon on top of berries, mix together and set dish aside.
In a medium bowl, mix together the oats, flour, brown sugar and salt. Cut in the pieces of butter with a fork or by hand. Until mixture resembles coarse crumbs.
Bake in preheated oven for 35-40 minutes, until topping is light brown and filling is bubbly.
Serve warm with a scoop of vanilla ice cream, if desired.
Enjoy!