Combine oatmeal, flour, baking powder, and salt in a large bowl.
In another bowl, cream together the butter and sugar.
Add the egg and vanilla extract to the butter mixture and mix well.
Add the dry ingredients to the wet ingredients and mix until just combined.
Fold in the oatmeal and raisins (or chocolate chips) to finish the dough.
Scoop the dough by the spoonful onto a baking sheet.
Bake at 180°C (356°F) for 12–15 minutes.