In a medium bowl combine, rolled oats, oat flour, brown sugar, cinnamon and salt. Add cubed butter to the bowl. Cut with a pastry cutter (or 2 knives, or the back of a fork) until well combined and mixture resembles coarse meal. Optional: add nuts to the crumb mixture if using and stir well. Chill in the refrigerator until needed
Preheat oven to 350°F.
To peel the peaches: bring a large pot to a boil. Score an ‘x’ on the bottom of the peach. Place peaches into the boiling water for 30 seconds. In a large bowl add ice and cold water. Spoon the peaches out of the boiling water and plunge them in the ice water bath and let cool. Peel the skin off the peaches than cut into ¼-1/2-inch slices and place in a large bowl.
To the peaches add lemon juice, vanilla extract, cinnamon and cornstarch. Stir well to combine.
Grease a 9x9 baking dish. Add the peaches to the greased pan and spread as evenly as possible, lightly pressing down.
Spread crumbs evenly over peach mixture and pat down. Bake for 45- 50 minutes until peaches are soft, and the crumble is lightly browned.
Take out of the oven and let cool for 10-15 minutes before serving with vanilla ice cream or whipped cream.