Combine flax seed and water to make flax egg, set aside to thicken. Preheat oven to 350°.
Stir together oats and baking soda in small bowl, set aside.
In a large bowl, using a mixer or spatula, combine softened butter, peanut butter, sugars and vanilla. Beat until smooth. Add flax egg and mix well.
Add oats and baking soda, and once combined, fold in chocolate and candy pieces. Batter will be thick.
Use large ice cream scoop or 1/4 measuring cup to scoop cookies onto sheet, leaving about 2 inches between cookies.
Bake for 15-20 minutes, or until golden and puffy. Allow to cool on sheets for another 20 minutes, then remove to wire rack to cool completely.
Store cooled cookies in an airtight container up to two weeks.