
Preheat the oven to 375℉.
In a large mixing bowl, stir together the rhubarb, strawberries, sugar and cornstarch. The fruit will become think and turn to a syrup.
On a lightly floured surface, roll out the pie crust dough to a 12" circle and place in the 9" deep dish pie plate. Trim and flat the edges. Pour the fruit mixture into the pie shell.
In a small bowl, stir together the flour and brown sugar. Add in the oatmeal and cinnamon. Cut in the butter until it looks crumbly.
Spread the flour mixture over the fruit, but stop a 1/2" from the pie's edge. Bake for 50-55 minutes. Check the pie to make sure the top isn't burning. Cover the pie with a piece of tin foil and bake for another 20 minutes.
If the pie still isn't fully set, bake for another 20 minutes. The pie should be bubbly and the streusel should be golden brown. Remove the pie from the oven and cool on a baking rack. Cool completely before serving. Serve with whipped cream or vanilla ice cream.