In a large bowl, cream together the butter, sugar and egg. Add the flour, baking powder, baking soda, and oats. Mix together until well combined. Stir in the nuts, chocolate chips and raisins.
Put 24 rounded tablespoonfuls of the mixture onto a large greased cookie sheet. Transfer to a preheated oven, 350 degrees and bake for 12 minutes or until the cookies are golden brown.
Remove the cookies from the oven, then transfer to a wire rack and let cool. While they are cooling, put the chocolate pieces into a heatproof bowl over a pan of gently simmering water and heat until melted. Stir the chocolate, then let cool slightly. Use a spoon to drizzle the chocolate in waves over the cookies, or spoon it into a piping nozzle and pipe zigzag lines over the cookies. Store in an airtight container in the refrigerator before serving.