Place all ingredients in a high-speed blender and mix until smooth. You'll need to blend it longer if the oats are making it gritty. (See notes if you don't have a high-speed blender.)
Notes
If you don't have a high-speed blender, you'll need to add the oats to the machine first and pulse them a bit to break them up. Then add the remaining ingredients and blend until smooth.Don't use steel-cut oats here, because they're too coarse and won't blend in well.See the article that goes with this post for more information about substitutions.