Preheat the oven to 350°F.
Combine rhubarb, strawberries, sugar, and 1½ tablespoons of flour.
In a separate bowl, mix together flour, oats, brown sugar, and cinnamon. Pour in the melted butter and vanilla. Mix to combine.
Spread the rhubarb mixture evenly in the baking dish. Sprinkle the oat mixture over the top.
Bake 45 minutes or until golden and bubbly.Let cool slightly before serving. Store leftovers in an airtight container in the refrigerator for 3-4 days.