Make the peach filling: In a small bowl, combine 2 to 3 ripe peaches (diced) with ¼ teaspoon cinnamon. Stir and let sit for 10 minutes while you prepare the oats.
Mix the overnight oats: In a medium bowl or storage container, stir together 1 cup rolled oats, 1 cup Greek yogurt, 1 cup milk, 2 teaspoons maple syrup, 1 teaspoon vanilla extract, and ½ teaspoon ground cinnamon until well combined. Fold in half of the peaches and their juices. Reserve the remaining peaches for serving. Cover and refrigerate overnight, or for at least 4 hours.
Yogurt topping: Shortly before serving, stir together the remaining ½ cup Greek yogurt, 1 teaspoon maple syrup, and ¼ teaspoon vanilla extract until smooth.
Assemble: Give the oats a good stir. If they seem too thick, loosen them with a splash of milk. Divide half of the reserved peaches between two bowls or jars. Spoon the oats on top, then finish with the remaining peaches, a generous dollop of vanilla yogurt, and a sprinkle of crushed graham crackers and chopped pecans. Serve immediately while the topping is still crunchy.
