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Preheat oven to 400 degrees.
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Generously grease a 9" x 13" pan with butter.
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Mix peaches with sugar, 1 tsp cinnamon, cornstarch, salt, and vanilla.
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Mix flour, oats, brown sugar, and melted butter. Press half of flour mixture into the bottom of the pan. Pour peaches into pan.
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Sprinkle with the remaining flour mixture.
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Bake for 30-35 minutes until golden brown. Serve warm or let cool completely.