Heat oven to 350°F. In large bowl, beat brown sugar and butter with electric mixer on medium speed, scraping bowl occasionally, until light and fluffy. Beat in vanilla and eggs until well blended.
On low speed, beat in all-purpose flour, whole wheat flour and baking powder, scraping bowl occasionally, until well combined. Stir in oats.
On ungreased cookie sheets, drop dough by heaping tablespoonfuls 2 inches apart. Flatten cookies to about 1/2-inch thickness.
Bake 12 to 14 minutes or until light golden brown. Cool 1 minute; remove from cookie sheets to cooling racks.