Add the old-fashioned oats and skim milk to a small saucepan.
Bring the mixture to a gentle boil over high heat.
Reduce the heat to medium.
Cook for 4–5 minutes, stirring frequently, until the oats become tender and creamy.
Remove from heat and cover.
Let the oatmeal stand for 2 minutes.
Transfer the oatmeal to a serving bowl.
Top with Greek yogurt, chopped apple, and walnuts.
Serve warm and enjoy.