In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add brown sugar, baking powder, baking soda, and salt. Beat on medium speed until combined, scraping the sides of bowl occasionally. Beat in eggs and vanilla until combined.
Beat in as much flour as you can with the mixer. Stir in any remaining flour with a wooden spoon. Stir in oats, mixed fruit bits, and nuts.
Bake in a 350 degrees degree F oven for 10 to 12 minutes or until edges are lightly browned. Let cookies stand on cookie sheet for 1 minute. Transfer cookies to a wire rack; let cool. Makes about 3 dozen cookies.