In a bowl, combine the melted butter and sugar, and mix well with a whisk.

Add the egg and mix well with the whisk.

Switch to a spatula, add the all-purpose flour, and gently mix.

Once the flour is incorporated, add the oatmeal in two batches, gently mixing each time.

Line a baking sheet with parchment paper, drop spoonfuls of dough onto it, and shape them into flat circles with wet fingers.

Bake in a preheated oven at 350°F for 15 minutes or until done!