Preheat the oven to 350°F. Line a rimmed baking sheet with parchment paper or a silicone baking mat.
In a large mixing bowl, combine ground flax with water. Set aside until thickened, 4-5 minutes.
Add coconut oil, brown sugar, and vanilla to flax mixture. Mix until smooth.
Add oats, flour, baking powder, cinnamon, and salt. Mix until everything is combined. Gently fold in walnuts and cranberries. The dough will be wet and sticky.
Drop dough onto the prepared baking sheet. Gently flatten with a fork.
Bake until cookies are golden around the edges, 13-15 minutes.
Remove from oven.
Allow cookies to cool for at least 30 minutes before serving.