Preheat the oven to 350°F (175°C). In a mixing bowl, combine sliced peaches, granulated sugar, lemon juice, and cornstarch. Toss to coat evenly.

In another bowl, mix together oats, flour, brown sugar, melted butter, and cinnamon until crumbly.

Spread the peach mixture in a baking dish and top with the oat mixture. Bake for 30 minutes or until the topping is golden brown and the peaches are bubbly.