Preheat the oven to 375 degrees F (190 degrees C).
In a saucepan, combine the raspberries, cherries and sugar. Cook over medium heat until soft. Remove from the heat and set aside. In a separate bowl, mix together the oats, brown sugar, cinnamon and melted butter.
Use a slotted spoon to spoon the fruit into the pie crust. Try not to add too much juice. Sprinkle the oat mixture over the top.
Bake for 10 minutes in the preheated oven, or until oats are toasted and crisp.