In a large mason jar, combine oats, milk, 1/2 cup yogurt, agave, and vanilla. Screw lid on and shake to combine. Let chill in fridge overnight or at least 8 hours.
When ready to serve: In a small bowl, combine blueberries, strawberries, sugar, and lemon juice. Let stand 5 minutes until berries begin to release their juices.
Remove oats from fridge and divide between 3 serving glasses. Top each with remaining 1/2 cup greek yogurt and top with berries. Serve overnight oats immediately and enjoy!