Bring water to a boil in a large pot. Stir oats into boiling water and cook at a boil until the mixture begins to thicken, about 5 minutes. Season oats mixture with salt; stir.
Reduce heat to low and simmer oats until beginning to soften, about 20 minutes.
Stir milk, brown sugar, melted butter, and cinnamon together in a bowl with a whisk until smooth; stir into the oats mixture. Continue cooking the oats until tender, 10 to 15 minutes more.
Stir cranberries into the oatmeal to distribute throughout the mixture; cook just until the berries are warmed, 2 to 3 minutes.