Stir together flour, oats, salt, and yeast in a large bowl.
Add in water and stir to combine. The dough should be very wet. Much too wet to work with.
Cover dough and let it rise at room temperature for 12-16 hours.
After rise, turn dough out onto a heavily floured surface and fold the dough over itself a few times, using lots of flour, until the dough holds its shape in a ball. You might need a cup or two of flour to get it to hold its shape.
Line a clean dish towel with flour and flaxseed if you’re using it. Transfer dough ball to the towel with the seam of the dough facing down toward the towel. Cover loosely and let rise a second time for 30 minutes.
Preheat oven to 500 degrees Fahrenheit for 30 minutes with the bottom part of a large cast iron dutch oven in the oven so it gets very hot. Don’t preheat the lid.
After 30 minutes pull out pan and carefully roll dough ball into pan so now the seam is facing UP. Cover pot and bake at 500 degrees for 30 minutes.
Remove lid carefully after 30 minutes (steam will rush out so be careful) and return to oven uncovered for 18-20 minutes until dough is nicely browned.
Carefully remove bread from pan and let cool for 30 minutes before slicing.
Let bread cool completely before storing in bags. You can store on the counter for a day or two if you are going to use it immediately or store in the fridge for 7-10 days.