In a medium bowl whisk together starter, water, honey, and olive oil until starter and honey have dissolved and mixture is bubbly.
In a large bowl or the bowl of a stand mixer whisk together flours and salt. Add wet ingredient mixture to dry ingredients and mix to form a shaggy dough.
Knead dough until soft, supple, and slightly tacky (but not sticky). This may take about 6-8 minutes by hand or 5-6 minutes in a mixer with dough hook on medium-low speed.
Transfer dough to a clear container or bowl. Mark the top of the doughs position on the side the container with a washable maker or rubber band and cover.
Leave dough to rise at room temperature until it has increased in volume by about 50-100%. This may take around 4-6 hours but this depends on the ambient temperature and the strength of your starter. For faster rising or if room temperature is below 65° proof in the oven with the light on (leave yourself a sticky note on the oven so you don’t forget it is in there).
Fill a small bowl with water and use it to wet your hands. Punch down dough then gently release from the sides of the container and transfer to a lightly floured work surface. Divide dough into 3 equal portions with a knife or bench scraper. (For best results, weigh portions for accuracy). Pinch and pull the edges of each portion of dough into the center to form into balls.
Working one at a time, roll balls into 4x7-inch rectangles. Starting from one of the short edges, roll each into a tight cylinder. Stack cylinders side-by-side in a lightly greased 8.5x4.5-inch loaf pan. Cover and allow to rise until about doubled in volume or about 1-inch over the rim of the pan, this may take around 3-6 hours.
When ready to bake preheat to oven 350°. Brush top of loaf with milk. Sprinkle with oats if desired. Bake 40-50 minutes until golden brown and internal temperature is at least 190°.
Allow to cool in pan for 5 minutes. Remove to a wire rack and cool completely before slicing.