Preheat the oven to 375°F. Lightly butter an 8- or 9-inch-square baking dish or 9-inch pie plate.
In a large bowl, combine fruit, 1/3 cup sugar and 1/4 cup of the flour and toss until evenly coated. Transfer to the buttered baking dish.
To make topping, stir together oats, remaining 1/4 cup sugar and 1/2 cup flour. Using a pastry blender or 2 knives, cut butter into oat mixture until well-combined. Spread topping over fruit, pressing down slightly.
Bake until the top is golden brown and the fruit is tender and bubbly, about 40 minutes. Let cool 10 minutes and serve warm.