Preheat the oven to 400°F. Butter a 9x9 baking pan, and set aside.
Mix the peaches, granulated sugar, 2 tablespoons flour, cornstarch, 1 teaspoon cinnamon and salt together in a large bowl until well combined. Pour the peach mixture onto the prepared pan and set aside.
In the same large bowl, cream together the brown sugar, remaining ½ cup flour, butter, and remaining teaspoon of cinnamon until it forms a wet, clumpy mixture. Add the oats and stir to form a coarse mixture.
Crumble the oats on top of the peaches in an even layer. Bake for 20 to 25 minutes, until the oats start to turn a light golden color around the edges. Remove from the oven and let rest for 10 minutes to cool and set up.
Serve warm, with vanilla ice cream if desired.