In a medium mixing bowl, combine the rolled oats, Greek yogurt, milk, honey, vanilla extract, chia seeds, cinnamon, and salt. Stir vigorously until the yogurt is fully incorporated and no clumps remain.
If using fresh berries, chop strawberries into bite-sized pieces and gently fold half the mixed berries into the oat mixture. Reserve the remaining berries for layering.
Divide the mixture evenly between two mason jars or airtight containers. If you folded berries into the base, layer the reserved berries on top. If using frozen berries, place them directly on top of the oat mixture in each jar.
Seal the containers tightly and refrigerate for at least 8 hours or overnight. The oats will absorb the liquid and soften to a creamy, pudding-like consistency.
In the morning, give the oats a good stir to distribute the berries and yogurt evenly. Add a splash of milk if you prefer a thinner consistency.
Top with additional fresh berries, granola, nuts, or a drizzle of honey if desired, and enjoy cold or at room temperature.