Adjust oven rack to center of oven and preheat to 350°F. Wash chicken legs and blot dry. Place legs on work surface. Starting at top of leg, slip your fingers under the skin to loosen. Peel off skin carefully, using a pair of kitchen shears to snip the skin and any connecting membrane to avoid tearing the meat; discard skin.
Trim off any membrane or bits of fat that may still be attached to leg. Lightly grease bottom of 9x13x2-inch ovenproof baking pan with cooking spray. Arrange drumsticks in single layer in pan. Arrange potatoes between and around drumsticks. Place a single layer of onion slices over drumsticks and potatoes. Spoon tomatoes over onions. Sprinkle oregano on top; season with salt and pepper. Drizzle with olive oil.
Cover pan with foil and bake for 25 minutes. Remove foil and baste with pan juices. Continue baking, basting frequently with pan juices, until drumsticks are very tender and potatoes are cooked, about 25 minutes. Transfer drumsticks to a platter. Spoon tomatoes, onions, and pan juices over top. Garnish with parsley and serve.