In a medium mixing bowl, combine flour, cocoa powder, baking soda, and salt. Set aside.
In a very large mixing bowl, beat butter with an electric mixer on medium speed for 30 seconds.
Add the sugar and beat mixture until combined, scraping sides of bowl occasionally. Beat in egg, water, and vanilla until combined. Beat in as much of the flour mixture as you can.
Using a wooden spoon, stir in any remaining flour mixture and the rolled oats. Stir in chocolate pieces and peanut butter flavored pieces.
Drop dough by rounded tablespoons 2 inches apart on an ungreased cookie sheet. Bake in a 350 degree F oven for 10 to 12 minutes or until the edges are set. Transfer cookies to a wire rack and cool. Makes 48 cookies.