Heat the oven to 220 C / 425 F with racks in the upper and lower thirds. Line two 33 x 46 cm / 13 x 18 inch sheet pans with parchment.
In a large bowl, whisk 2 tbsp olive oil, the zest of both lemons, the juice of 1 lemon, 3 minced garlic cloves, the oregano, 1 tsp salt, and 1/2 tsp pepper. Add the dry thighs and rub the marinade over the meat and under the skin. Let them sit for 15 minutes while the oven heats. Only half the garlic goes in now, because minced garlic scorches in a 220 C oven and turns bitter.
In a second bowl, toss the potatoes with 2 tbsp olive oil, 3/4 tsp salt, and 1/4 tsp pepper. Divide them between the two pans, cut-side down. Set 4 thighs skin-side up on each pan, leaving a little space around each piece.
Roast for 25 minutes, swapping the pans between racks after about 12 minutes, until the chicken skin starts to turn golden.
Toss the green beans with 1 tbsp olive oil and 1/4 tsp salt. Pull the pans out, scatter half the beans on each pan, and roast for 15 minutes more. The chicken is done when the skin is deep golden, a fork slides easily into a potato, and a thermometer in the thickest part of a thigh, not touching bone, reads 175 F / 80 C.
Stir the remaining garlic and the parsley into the juice of the second lemon. Spoon it over the hot chicken and vegetables, and serve right away with the pan juices.