Add the oat milk, ¼ avocado, oats, matcha, and agave to the blender, and puree until smooth. Let the blended mixture sit for 5 minutes.
Add more avocado (if you want) and the ice and blend again.
Video
Notes
Letting the blended mixture sit for a few minutes allows the blended oats to absorb some of the moisture and soften. That way, when you add the ice and blend again, you'll get the smoothest results.