Preheat oven to 350°F and line two baking sheets with parchment paper. In a medium bowl, whisk together oats, flour, baking soda, and salt until evenly combined.

In a large mixing bowl, cream the softened butter and brown sugar together for 2-3 minutes until the mixture becomes light and fluffy.

Beat in the egg, fresh lemon juice, lemon zest, and vanilla extract until fully incorporated.

Gradually fold the dry ingredients into the wet mixture using a wooden spoon or rubber spatula. Mix just until the flour disappears and you have a cohesive dough.

Using a cookie scoop or tablespoon, drop rounded portions of dough onto prepared baking sheets, spacing them about 2 inches apart.

Bake for 10-12 minutes, watching for edges that turn golden brown while centers still look slightly soft and pale.

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