Preheat oven to 350°F. Line two cookie sheets with parchment paper.
In a medium bowl combine the oats, flour, baking soda, baking powder and salt; mix well.
In a large bowl, beat the butter and sugars with an electric mixer until light and fluffy, about 3 minutes. Add the peanut butter and beat until smooth. Add the eggs, one at a time, beating well after each addition.
Add the flour mixture and beat on low speed until blended. Stir in the dried cranberries.
Divide the dough into quarters. Pat each quarter into an 8-inch circle on a prepared cookie sheet. Sprinkle the cookie with 2 tablespoons of the green decorating sugar.
Cut the dough into 16 wedges, about 1 1/4-inches wide. Gently lift up the end of each triangle and place a pretzel round underneath as the "trunk" of each cookie tree. Press the dough around the pretzel. Bake until golden brown and firm to the touch, about 15 to 17 minutes. Let cool on the pan for about 5 minutes. Transfer the individual cookies to a wire rack and let cool completely.
Repeat with the remaining cookie dough.
When the cookies are completely cool, spoon the vanilla frosting into a freezer weight ziptop bag and seal. Snip a very small corner from the bag. Pipe the frosting onto the trees like a garland. Decorate with small clusters of the decorating balls.