In a small bowl, stir brown sugar, hot water, and cinnamon until the sugar dissolves completely.
Brew your espresso shots and pour them into a cocktail shaker or a jar with a tight-fitting lid.
Add the brown sugar cinnamon syrup and ice cubes to the shaker with the espresso.
Shake vigorously for about 15-20 seconds until the mixture is well-chilled and frothy.
Fill a tall glass with fresh ice, pour in the oat milk, then strain the shaken espresso mixture over the top.
Stir gently to combine the layers and drink immediately.