In a large bowl, combine flour, oats, sugars, cinnamon, and sea salt. Toss in the cold butter. Use your hands to work the butter in, pressing the pieces with your fingers until they're about pea-sized and fully incorporated.
1 cup all purpose flour, ¾ cup old fashioned oats, ½ cup brown sugar, ½ cup granulated sugar, ½ tsp cinnamon, ¼ tsp sea salt, 10 Tbsps cold butter