Step 1On a board, combine the olives, garlic, chiles, zest and oregano and finely chop them. Place the mixture in a bowl along with the oil, orange juice, vinegar and lemon juice. Set aside to give the flavors time to infuse. This makes about 1 ½ cups gremolata.
Step 1Trim the chicken thighs of scraggy bits of skin. Pierce the undersides with a sharp knife and put in a non-reactive dish. Add the oregano, garlic, orange juice and zest, ¼ cup of oil and the pepper. Mix with your hands, cover, and put in the fridge for a few hours (overnight is even better).
Step 2Heat the oven to 375 degrees. Remove the chicken from the marinade, shaking off any excess. Heat the remaining oil in a large ovenproof sauté pan or shallow cast-iron casserole large enough to fit the chicken in a single layer. Brown the chicken, in batches, on both sides, finishing skin-side up. Scatter with sea salt flakes and roast for 20 minutes.
Step 3Lay some of the orange slices under the chicken and the rest on top. Spoon the cooking juices over the oranges, then sprinkle a little sugar over the slices. Roast until the chicken and oranges are fully cooked, about 20 minutes more. Scatter the gremolata on top. Serve straight from the sauté pan or casserole.