Preheat the Oven: Begin by preheating your oven to 350°F (175°C).
Prepare the Dough: In a bowl, mix flour, rolled oats, baking soda, and salt. Set aside.
Cream the Butter and Sugars: In a separate bowl, beat the softened butter, granulated sugar, and brown sugar until creamy and fluffy, about 2 minutes.
Add Egg and Vanilla: Mix in the egg and vanilla extract until fully incorporated.
Combine Dry and Wet Ingredients: Gradually add the flour mixture to the butter mixture, stirring until just combined.
Fold in Toppings: Gently mix in the chocolate chips, mini marshmallows, and crushed graham crackers until evenly distributed.
Scoop the Dough: Use a cookie scoop or spoon to place dough balls on a baking sheet lined with parchment paper, spacing them about 2 inches apart.
Bake: Bake for 10-12 minutes or until the edges are golden brown.
Cool and Enjoy: Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.