Boil a pot of water and blanch the stinging nettle in it for 90 seconds. Remove the nettle and place in a cold water bath. Pick the leaves from the stems and wring them out in a kitchen towel to remove excess water. Finely chop the leaves by hand, or puree in the food processor.
In a large bowl, whisk together flour, yeast, oregano and salt. Add the warm water and chopped stinging nettle and stir until a sticky dough is formed. No kneading is necessary, but if the dough is too sticky to handle you can add a bit more flour.
Liberally oil a separate large bowl. Place the dough in this bowl, turning it to coat it well in the oil, and wrap tightly with plastic wrap. Move it to the fridge and allow to rise slowly for at least 12 hours, but preferably 24 hours.
When you're ready to bake, liberally oil a 9x13 baking pan. Transfer the dough to the pan and stretch it with your fingertips until it reaches the corners. Cover and allow to rise in a warm spot until doubled, 1-2 hours.
Preheat the oven to 450. Use your fingertips to make indentations all over the top of the focaccia, then drizzle with more olive oil and top with a sprinkle of flaky salt. Bake, turning once, until browned and cooked through, 25-30 minutes. Allow the focaccia to cool on a rack and enjoy warm.