Brew espresso using your preferred espresso making method. See my 5 Ways To Make Espresso At Home blog post for step by step instructions of the various espresso making method! To a shaker cup or mason jar, add horchata syrup, espresso, and ice.
Add the lid and shake vigorously until the entire cup is cold to the touch.
Pour the mixture into a separate cup (if using a shaker cup/cocktail shaker).
Add oat milk over the top, and enjoy.