The best way to keep your drink from becoming watered down is to use very cold milk and large ice cubes. The smaller the ice, the faster it will melt. When pouring the shaken espresso into your drinking glass, be sure to strain the ice cubes in the jar. These ice cubes will most likely be broken up into smaller pieces from shaking the jar and will melt too quickly if adding to the glass. You can also add the oat milk before adding the ice. This will further chill the freshly brewed espresso, so all the liquid is cold before adding the ice.