Prepare the hot water. Room temperature water is fine, but the hot water allows the matcha to dissolve more easily as well. Avoid using cold water, as this can cause the matcha to bind together in big clumps.
In a small bowl, combine your matcha and water and whisk moderately for about 30 seconds or until a smooth, dark green liquid is achieved. A ceramic or textured bowl will help break up any lumps while mixing and accelerate the process.
Pour in honey, agave, or other sweeteners into the mix, and continue to whisk until smooth. Liquid sweeteners mix better than granulated sugars and add to the velvety texture of the matcha.
Add ice and oat milk to a fresh glass of your choosing. Pour the matcha mixture over the iced oat milk and gently stir. While it may seem faster and easier to just create the matcha mixture in the drinking glass and top it with ice and oat milk, this method will likely leave bitter matcha lumps at the bottom of your drink and unevenly mix with the added oat milk and ice on top.
For an extra topping, take some additional oat milk (or oat milk creamer) and froth it in a separate cup for 30 seconds. Then, pour it over the latte, sprinkle some cinnamon on top, and enjoy your drink!