Combine the oats, ¾ cup of the almond milk, water and salt in a small saucepan. Bring it a boil over high heat, then reduce the heat to medium-low and simmer until the oats are tender, about 4 minutes.
Remove from the heat and transfer the oatmeal mixture to a bowl and allow to cool for about 10 minutes. Stir in the remaining ¼ cup almond milk and 1 teaspoon of the maple syrup. Cover with plastic wrap and refrigerate overnight.
In a small bowl, toss the chopped plum, remaining 1 teaspoon of maple syrup and cinnamon in a bowl; let stand for 5 minutes. Serve the chilled oatmeal topped with the fruit and any accumulated juices.