Start heating your oven to 200 C / 400 F / Gas Mark 6
Grease an 8 inch (20 cm) round pie pan.
In a bowl, mix the oats, flour, sugar, and baking soda with a pinch of salt. Mix in the melted butter.
Spoon into the pie pan and press down (don't line the sides of the pie pan, just the base.) Mark the edge with a fork. Bake until golden, about 15 minutes. Mark into 8 wedges while still hot.
When cooled, slice and remove from pan and serve anytime.