Why would you object to calling it oat milk? It's simple, descriptive, doesn't deceive anyone. It's not a coincidence that in most (all?) languages coconut milk is a just coconut + milk in that language. (Using coconut milk as an example since it is a much older product than oat milk)

Oat milk may be a recent invention, but alternative milks aren’t. E.g., almond milk (“almonde mylk”) dates back to the Middle Ages as a dairy milk substitute.