Pat the chicken dry with paper towels, and season all over with salt and pepper.
Place a large flat-sided skillet or Dutch oven (preferably enameled cast iron or stainless steel) over medium-high heat, and add the oil.
Once the oil is hot shimmery, add the chicken, skin-side down, leaving some space so they’re not touching. Cover with a splatter screen if you have one. Sear the skin for approximately 5-7 minutes without moving it. Turn the heat down or briefly move the pan off the burner if you feel like it may burn, but don't try to flip the chicken too soon or the skin will stick to the pan. The skin will release on its own when it's ready. Once the skin is brown and crispy, use a spatula to flip the chicken and then cook for an additional 2-3 minutes. Using kitchen tongs, remove from the pan and set aside on a plate, skin-side up to preserve the crispy skin.
Add tomatoes, harissa, sugar, the leaves from 1 sprig oregano, and 1/2 teaspoon salt to the same pan (don't wipe it down; leave the brown bits). Increase the heat to medium-high and cook, stirring and carefully breaking up the whole peeled tomatoes with a spatula or wood spoon, until the sauce has started to thicken slightly, 8–10 minutes.
Nestle chicken thighs, skin-side up, back into skillet over the sauce.
Reduce heat to medium-low and cook until an instant-read thermometer inserted into the thickest part of the thighs near the bone register 165 degrees F), and tomato sauce has thickened up a bit more, 6–8 minutes.
Remove from the heat and let the the chicken rest for 5 minutes. Crumble the feta over the chicken, along with the leaves from 2-3 sprigs of oregano.
Serve, optionally with torn crusty bread to soak up the sauce.
Leftovers will keep in an airtight container in the refrigerator for several days, but the skin will lose it's crispiness once you chill it. It still tastes good!
I recommend buying a block of feta in brine. Pat it dry gently before using, and cut into cubes. Crumble the cubes, leaving some larger pieces. I used this pan. In order to get crispy skin, use something that's NOT nonstick.