Preheat the oven to 400°F. In a large bowl, toss potatoes and onions with olive oil; season with salt and pepper.
Spread in 1 layer on a roasting pan, cut sides down so no vegetables are overlapping. Roast until the potatoes are browned and just tender (50-60 minutes). Remove and toss with lemon juice, zest, oregano, and black olives.