Preheat the oven to 350°F (175°C).

In a large mixing bowl, cream together the softened butter and brown sugar until smooth and fluffy.

Add the egg and vanilla extract, beating until well combined.

In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon.

Gradually mix the dry ingredients into the wet ingredients until fully combined.

Stir in the chopped rhubarb and rolled oats until evenly distributed.

Drop spoonfuls of cookie dough onto a parchment-lined baking sheet, spacing them about 2 inches apart.

Bake for 10-12 minutes, or until the edges are lightly golden and the centers are set.

Let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.