Add the rolled oats, water, milk, and salt to a small saucepan over medium heat or to a microwave-safe bowl.
If using the stovetop, bring to a gentle boil then reduce to a simmer and cook, stirring occasionally, until thickened, about 5 minutes.
If using a microwave, cook on high for 2 minutes, stir, then continue in 1-minute increments until thick and cooked through.
Remove from heat and whisk in the protein powder until fully combined and smooth.
Top as desired and serve immediately.