Choose an 8- to 12-ounce jar or airtight container. Add the rolled oats, flax or chia seeds and cinnamon into the jar and give them a quick stir to distribute the spices and seeds evenly.
Pour the almond milk over the oat mixture and stir briskly so the oats are moistened. Press the oats down lightly with the back of a spoon, then let the jar sit on the counter for 3–5 minutes so the seeds begin to hydrate.
Meanwhile, in a small bowl, whisk the Greek yogurt and vanilla protein powder until smooth and lump-free. If the yogurt is very thick, you can thin it with 1–2 teaspoons of almond milk to make it spreadable.
Spoon the yogurt-protein mixture over the soaked oat layer, smoothing the top so the two layers stay mostly separate. This creates a creamy contrast that keeps the oats from becoming too dense.
Top the jar with the frozen berries. Press them gently into the yogurt so they nestle against the layers; the berries will thaw and release a little juice into the yogurt overnight for extra flavor.
Seal the jar and refrigerate for at least 6 hours, ideally overnight. Before eating, give the jar a quick stir or shake to combine layers as you like, or enjoy as a layered parfait.