Preheat the oven to 350 degrees. Butter a 10-inch cast iron skillet.Make the streusel by combining the flour, oats, brown sugar, baking powder, and salt together in a bowl.
Add the butter and press and pinch it into the flour mixture with your fingers until evenly moistened and crumbly. Add the chopped pecans and mix well; set aside.
Make the strawberry buckle by whisking together the flour, baking powder, salt, and baking soda in a small bowl.
In a large bowl, beat the butter and sugar together until fluffy and creamy, 2 minutes.
Add the eggs, one at a time, and then the vanilla extract; beat until creamy and smooth.
Add the flour mixture and the buttermilk in increments, beginning and ending with the flour mixture, until just combined.
Gently fold in the strawberries. Pour into the prepared cast iron skillet. Top evenly with the streusel mixture.
Place into the oven and bake for 35-40 minutes, or until a tester inserted in the center comes out with moist crumbs attached.
Serve warm with a scoop of ice cream or whipped cream, if desired. Enjoy!