Preheat oven to 350 degrees F (175 degrees C). Grease cookie sheets.
In a large bowl, cream together the butter, white sugar, and brown sugar until smooth. Beat in the egg, then stir in the vanilla. Combine the flour, baking powder, and baking soda; stir into the creamed mixture. Mix in the oats, white chocolate, and macadamia nuts until evenly distributed. Drop by teaspoonfuls onto the prepared cookie sheets.
Bake for 10 to 12 minutes in the preheated oven, or until edges are toasted. For crisper cookies, bake longer. If you leave the cookies on the cookie sheet for a few minutes before transferring to cool on wire racks you will have a softer bottomed cookie.