Preheat the oven to 350F. Line a baking sheet with parchment paper or a silicone baking mat.
In a medium bowl, whisk together flour, baking soda, and salt.
In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat butter and peanut butter on medium-low speed until smooth and well combined. Add the sugars and beat on medium speed until light and fluffy, about 2 minutes.
Beat in the eggs and vanilla on medium speed until well combined, stopping to scrape down the bowl as needed.
With the mixer on low, add the flour mixture, mixing just until almost combined. Scrape down the bowl and add the rolled oats. Mix just until combined.
Scoop roughly 2 tablespoons of cookie dough onto the baking sheet, placing each cookie about 2 inches apart.
For gooey cookies, bake for about 12 minutes or until the edges are set but the center still appears wet or shiny. For crispier-edged cookies, bake for about 15 minutes or until the edges and tops are lightly browned and the center is no longer shiny. Let cookies cool on the pan for 5 minutes. Remove and finish cooling on a wire rack.
Make sure your brown sugar is soft and moist for the best cookies. If your brown sugar has hardened, don’t worry, it’s an easy fix! I wrote a detailed post on how to soften brown sugar.
If you don’t have brown sugar, you can follow this post on how to make brown sugar.
Don’t overbake the peanut butter oat cookies, as they’ll continue to bake as they cool. The middle should still be soft when you pull them out.
Add a pinch of cinnamon for extra flavor.
Room temperature egg will mix more evenly into the creamed butter. Place your egg into a bowl of warm water for a couple of minutes. Doing so will quickly bring it to room temperature if you have forgotten to take it out of your fridge ahead of time.
Measure your flour correctly to avoid dry and dense cookies. Adding too much flour to the recipe is the most common mistake. The easiest way to measure flour is by using a scale. If you don’t have one, then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.