Preheat oven to 350 degrees F. Butter a 9 x 13 inch pan.
Mix together flour, baking powder and salt in small bowl. Set aside.
In a medium size bowl, mix the condensed milk with the lemon rind, then gradually mix in the lemon juice (mix with wire whisk). Set the lemon mixture aside.
In the large bowl of an electric mixer cream the butter. Add the sugar and beat well. On lowest speed gradually add the flour mixture. Scrape side or bowl and mix only until thoroughly mixed. Mix in the oatmeal. The mixture should be crumbly.
In prepared baking pan, add a little more than half of the oatmeal mixture. Pat the crumbs firmly and evenly .
Top the bottom crust with the lemon mixture, smooth out to all sides.
Sprinkle the remaining crumbs evenly over the lemon. Pat the crumbs gently. (It is okay if some of the lemon still shows through).
Bake for 30 to 35 minutes or until the crust is lightly colored.
Cool the cake completely in the pan. Then refrigerate it for an hour or longer.
When cool, cut around the edge of the pan with a small knife to release cookies. Cut into quarters. Cut quarters into small bars. Place on a serving plate. Cover with plastic wrap and refrigerate until ready to serve.