In a medium saucepan over medium heat, bring water, milk, salt, and cinnamon to a boil. Reduce heat to a simmer and stir in oats; cook, stirring occasionally, until thick and creamy, 6 to 8 minutes.
While the oatmeal is cooking, In a small bowl, toss strawberries with 2 tablespoons firmly packed dark-brown sugar. Let sit at least 5 minutes to bring out the juices.
Divide oatmeal between four serving bowls. Top each serving with 1 tablespoon sour cream. Divide the strawberries among each serving and sprinkle with remaining brown sugar.