Preheat your oven to 350°F (175°C) or preheat your grill to medium heat.
Carefully twist apart the Oreo cookies, separating the halves with the cream filling intact. Set them aside.
Place one marshmallow on top of each Oreo cream filling.
Arrange the Oreo marshmallow sandwiches on a baking sheet or grill pan, ensuring they are spaced apart for even heating.
Place the baking sheet or grill pan in the preheated oven or grill. Bake or grill for approximately 3-4 minutes, or until the marshmallows are puffed and golden brown. Keep a close eye on them to prevent burning.
Once the marshmallows are toasted, remove the baking sheet or grill pan from the oven or grill.
Immediately place one thin square of Hershey's chocolate bar on top of each toasted marshmallow.
Take the remaining Oreo cookie halves and press them gently on top of the melted chocolate, creating a sandwich with the marshmallow and chocolate in the center.
Allow the Oreo S'mores to cool slightly before serving. The chocolate and marshmallow will remain delightfully warm and gooey.
Serve these irresistible Oreo S'mores to your eager guests and enjoy the perfect combination of crunchy cookies, melted chocolate, and gooey marshmallow goodness.