Preheat your oven to 180°C / 350-375°F / Gas Mark 4-5.
Use room temperature butter to grease a 12 cup muffin tin or fill with patty pan liners.
Add oats to a large mixing bowl, followed by buttermilk, olive oil and egg. Stir the mixture to combine, then let sit for at 10 to 15 minutes.
Add self-raising flour, brown sugar, baking powder and salt to the muffin batter, stir to combine.
Pour the blackberries and mashed banana in and gently stir to combine.
Spoon the muffin mixture into the muffin tin evenly across the holes.
Oven bake the muffins for 30 mins, until a skewer inserted comes out clean with no wet batter.
Use an oven mit to transfer muffin tin to a wire track. Let muffins cool in tin for around 5 mins, before removing and placing on the rack.
Enjoy with butter, drizzled honey, additional blackberries and icing sugar.