In a skillet over medium heat, toast large-flake rolled oats and nuts (if using), stirring constantly, for 2 to 3 minutes or until golden and fragrant. Immediately transfer to a bowl and let cool. Stir in flax seeds (if using).
In a bowl, whisk together plain yogurt and vanilla yogurt.
Alternately layer oat mixture, yogurt and berries in four individual 1-1/2-cup (375 mL) reusable containers with lids or tall glasses. Seal with lids or cover with plastic wrap and refrigerate for at least 8 hours or up to 2 days.