In a mason jar or airtight container, combine oats, milk, and yogurt (if using).

Stir in diced apple, chia seeds, maple syrup (or honey), cinnamon, vanilla extract, and salt until well mixed.

Seal the container and refrigerate overnight (at least 6 hours).

In the morning, give the oats a good stir. If too thick, add a splash of milk to reach your preferred consistency.

Taste and adjust sweetness or cinnamon as desired.

Top with extra apple slices, a drizzle of maple syrup, chopped nuts, or a sprinkle of cinnamon before serving cold—or warm it gently in the microwave.