Preheat an oven to 425 degrees F.
Set the lamb leg in a large roasting pan, and season it well with sea salt and black pepper. Sprinkle the meat with the oregano and pour the lemon juice and wine over top. Drizzle the lamb with the olive oil and roast until the exterior is browned, about 20 minutes.
Add the water to the bottom of the pan and cover the lamb loosely with foil. Reduce the oven temperature to 315 degrees F, and roast the lamb for another 1 1/2 hours.
At this point, arrange the potato chunks and lemon quarters around the lamb and return the pan to the oven for another 2 hours, turning the potatoes at least once and basting the lamb with the pan juices. The lamb should be very tender. Remove the meat from the pan and set it aside to rest before slicing.
Increase the oven's heat to 425 degrees F.
Add the olives to the potatoes and return the pan to the oven until the potatoes are golden, about 20 minutes. Serve the vegetables with the lamb and with some pan juices spooned over the top.