Preheat your oven to 350°F (175°C).
In a large bowl, combine peanut butter and brown sugar until smooth.
Add the egg and vanilla extract, mixing well.
In a separate bowl, mix the rolled oats, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture.
Stir until fully combined; the dough will be thick.
Scoop out portions of dough and place them on a baking sheet lined with parchment paper.
Flatten each cookie slightly with the back of the spoon.
Bake for 12-15 minutes or until the edges are golden brown.
Let cool on the baking sheet for a few minutes before transferring to a wire rack.