In a large bowl, cream the butter and shortening with the sugars for about 2-3 minutes. Add eggs and vanilla and beat for another 1-2 minutes.
In a separate bowl, mix together the flour, baking soda and salt. Add to the creamed mixture and mix just until combined.
Stir in oats, chocolate chips and mini marshmallows.
Scoop onto cookie sheets that have been greased or lined with parchment paper or silicone baking sheets. Bake for 9-11 minutes or until cookies have slightly begun to brown on the edges.
Let cookies cool on baking sheets for a few minutes before transferring to wire racks to cool completely.