Preheat oven to 350°F and line a 9x9-inch pan with parchment paper. Set aside.
In a large bowl, combine oats, flour, brown sugar, baking powder, and baking soda. Pour melted butter over the mixture and mix together until crumbly, being careful not to overmix.
Measure out ½ of the mixture (about 2 cups), and set it aside to reserve for topping.
Pour the remaining half of the mixture into the lined baking dish. Press firmly into the bottom of the pan.
Bake 12-15 minutes, or until golden brown. Set aside to cool slightly while you make the lemon filling.
To a large bowl, add sweetened condensed milk, lemon juice, egg yolks, lemon zest, and salt. Mix until smooth and creamy.
Pour the lemon mixture over the top of the baked oat crust.
Evenly crumble the reserved oat mixture over the top.
Bake for 20-25 minutes, or until edges are golden brown.
Let come to room temperature. Then, refrigerate 1 hour before serving.