Vanilla Option: Stir 1½ teaspoons of vanilla extract or paste (or to taste) into the custard mixture right after you remove it from the heat.
Strawberry Option: Place 2 cups finely chopped fresh strawberries, 2 teaspoons sugar, and 1 teaspoon bourbon in a small bowl, and toss to combine. Let the berries macerate at room temperature for about 1 hour. Lightly smash or pulse the berries with a stick blender or food processor to break up any large chunks. Refrigerate the berries while the oat milk mixture refrigerates. Stir the berries into the oatmilk mixture before processing it in your ice cream maker, or add the berries as it churns.
Other Berry Options: Follow the directions for the Strawberry Option, but you can substitute blueberries, blackberries, or raspberries. If using blackberries or raspberries, press the berries through a fine mesh sieve to remove the little seeds. For blueberries, lightly smash the berries and combine with the sugar and bourbon.