Add oats & flour mixture to butter/sugar mixture & combine. Make sure there are no butter chunks.
Press ½-¾ mixture on the bottom of an 8x11 (not 9x13) pan (there is a TON of crumb mixture, so I would recommend more than half pressed on the bottom b/c it gets messy with a lot on the top.)
Mix together the condensed milk, lemon juice, & zest. Spread onto the bottom layer of the crumb mixture.
Knead the remaining crumb mixture with hands, & crumble on the top of the lemon layer, but don't press down or you can ruin your lemon filling.
Bake for 20-25 minutes or until golden brown. Allow pan to rest for 30 minutes before cutting/serving. Delicious served cold from the fridge!