Beat butter at medium speed with an electric mixer until creamy; gradually add sugars, beating well. Add peanut butter, eggs, and vanilla; beat well. Add 4 1/4 cups oats, soda, and salt; stir well. Stir in 1 cup chocolate pieces and pecans. (Dough will be stiff.)
Pack dough into a 1/4-cup measure. Drop dough 4 inches apart onto parchment lined cookie sheets. Lightly press each cookie with fingertips into a 3 1/2-inch circle. Press 3 or 4 remaining chocolate chunks into each cookie and sprinkle with remaining oats.
Bake at 350° for 14 minutes (centers of cookies will be slightly soft). Cool 3 minutes on cookie sheets; remove to wire racks to cool.