Preheat the oven to 350?F (175?C).

In a mixing bowl, combine the flour, oats, baking soda, baking powder, salt, and cinnamon.

In another bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Fold in the diced rhubarb and chopped walnuts.

Drop spoonfuls of the cookie dough onto a baking sheet lined with parchment paper.

Bake in the preheated oven for 10-12 minutes or until the edges are golden.

Remove from the oven and let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.