Heat oven to 375 degree F. In a medium bowl, stir together flour, oats, baking soda, salt and baking powder.
In a large bowl, with an electric mixer, beat together butter, sugar and eggs until well blended and light in color, about 3 minutes. Add vanilla.
On low speed, beat in flour mixture (dough will be thick). With a spoon, stir in chocolate and walnuts.
Drop batter, using a 1/4-cup measure onto ungreased baking sheets, spacing about 2 inches apart. Bake cookies in 375 degree F oven for 15 to 17 minutes, rotating pans halfway, until cookies are slightly spread, golden and dry to the touch. Let cool on pans on rack for 1 minute; transfer cookies to rack to cool completely. Store cookies at room temp in airtight container for up to 2 weeks, or freeze for up to 3 months. Makes 22 cookies.