Using a mixer, mix the crisco for 2 minutes.
Add sifted powdered sugar one cup at a time. Once all of the powdered sugar is added, mix for 1 more minute, if it won't mix because it is too dry add oat milk 1 tablespoon at a time until fully encorpated. Scrape bowl.
If adding a flavor do this now, before adding additional oat milk. Add oat milk to frosting depending on desired thickness (1 tablespoon at a time). You may not need all of the milk, you may need more.