Preheat your oven to 180C/160 Fan/350F/Gas 4. Lightly grease two large baking sheets.
Cream the butter and both sugars together with a mixer until smooth. Add the oats, flour, bicarbonate of soda and a small pinch of salt and mix until thoroughly combined. If you are adding chopped nuts, chocolate chips or dried fruits you can add a handful at this stage – stir in gently to ensure that they are evenly spread through your mixture.
Take a walnut-sized spoonful of the mixture and roll into a ball and place on your baking sheet. Continue until you have used all the mixture, placing the biscuits well apart on the tray. The mixture should make 18-20 biscuits.
Bake for around 15 minutes, by which time the biscuits should be a lovely golden brown colour. Remove from oven and leave on the tray for 2-3 minutes, then remove to a wire rack until cooled.
Best enjoyed within two days – if they last that long!