In a large bowl, mix the water and sourdough starter together until fully combined.
Add the flour, salt, and oats. Mix until no dry bits remains. The dough will be sticky and shaggy.
Cover the bowl and let the dough rest for 20 minutes.
Perform stretch and folds every 30 minutes for the first 2 hours (4 rounds total).
Continue bulk fermentation (let the dough rise) until the dough has grown about 50% and feels elastic. This should take about 4 hours, depending on the room temperature.
Transfer the risen dough to a floured work surface. Shape the dough by gently pulling one side toward you and folding it into the center. Rotate the dough 90 degrees and repeat, continuing around the dough until it tightens into a round.
Lightly moisten the outside of the dough, roll it in 2/3 cup oats, then place it seam-side up in a floured banneton to proof. Proof the dough at room temperature for about 2 hours, or chill it overnight in the refrigerator (or for at least 4 hours).
Place a Dutch oven (with the lid) on the center rack of the oven and preheat it to 450°F.
Turn the dough out onto a sheet of parchment paper and score the top gently.
Place it into the preheated Dutch oven and bake it covered for 20 minutes, then remove the lid and bake it for another 22–25 minutes, or until the crust is deeply golden brown. Allow the bread to cool slightly before cutting into it. Enjoy!