To make the berry mixture, mix all of the ingredients in a bowl, so that the berries are evenly coated in the sugar, lemon juice, and corn starch. Spread the mixture evenly over the oatmeal base. Then, crumble the reserved oatmeal mixture over the berry layer for the oatmeal crumble topping.
Bake for 45-50 minutes, until the crumble on top starts to turn golden brown. You do not need to let them cool if you are serving in a bowl with vanilla ice cream and eating more like a cobbler. However, to ensure that they turn into bar form, cool in the fridge for 2 hours before cutting and serving (hopefully warmed back up with that scoop of vanilla!). Store bars in the fridge.