In a large saucepan, combine the oat milk, water, rice, sugar, and vegan butter. Add the cinnamon stick and stir well with a wooden spoon.
Place the saucepan over medium heat and bring the mixture to a gentle simmer. Stir occasionally to prevent sticking.
Continue cooking, stirring occasionally, for about 10-15 minutes or until the mixture starts to thicken. Add the vanilla and raisins.
Simmer the pudding for another 5-10 minutes or until the mixture is about 80% of your preferred thickness, or 70% if you intend to enjoy it chilled. Remove from the heat and continue stirring the pudding for about 5 minutes until it has cooled slightly.
Remove the cinnamon stick. Serve warm with a sprinkle of ground cinnamon, or place into an airtight container and into the fridge to chill.
Store leftover pudding in an airtight container in the fridge for up to 5 days.
Notes
Nutrition facts will vary depending on substitutions and brands. Cooking time will vary; stop cooking when the mixture is about 80% of desired firmness for warm pudding or 70% for chilled pudding.