In a food processor, combine the flour, oats, brown sugar, cinnamon and salt. Pulse to mix well. Cut butter into tablespoon sized pieces, add to the mixture and pulse until the butter is fully incorporated and mixture looks consistent.
Turn mixture into a bowl and using your fingers, bring together small crumbles my pinching the mixture together. Cover and keep in refrigerator if using the same day. Otherwise, place in a freezer bag and keep in the freezer.
Tip: Make a double batch and have extra on hand in the freezer for the next time you make a pie. This is a great step to do ahead of time.