Soak 200 grams of old-fashioned oats in a bowl with 100 grams of warm water and let sit for at least 1 hour.

Combine active sourdough starter, remaining warm water, soaked oats with soaking water, white bread flour, oil, honey, and salt in a large mixing bowl and mix until shaggy.

Cover the bowl and let dough rest for 1 hour at room temperature.

Perform a series of four 'stretch and folds' every 20-30 minutes to develop structure.

Allow the dough to ferment for 3-4 hours at room temperature, then refrigerate overnight for 10-12 hours.

Remove from fridge and let come to room temperature for 1-2 hours.

Divide dough into two or three portions, shape into loaves, and place in greased loaf pans to proof for 1-1.5 hours.

Preheat oven to 400°F (200°C) while loaves are proofing.

Score the tops of the loaves and bake for 35-40 minutes until golden brown.

Remove from pans and cool on a wire rack; brush with butter if desired.