In a medium bowl, combine 1 ½ cups Lucky Charms cereal with milk and water. Place in fridge for about 1 hour to let cereal flavors infuse into the liquid.
Strain the liquid into a medium saucepan, leaving the cereal behind. Bring liquid to a boil.
Add oats and reduce the heat to a simmer. Cook for 10-15 minutes, stirring occasionally, until oats have absorbed most of the liquid and desired consistency is reached.
Serve oatmeal in bowls and top with Lucky Charms marshmallows.