Preheat the oven to 400F. Rub the venison with the olive oil, 1 teaspoon of the pepper and 1/2 teaspoon of the chopped juniper berries, pressing the seasonings into the meat. Set the loin on a rack in a roasting pan and roast, basting frequently with the pan juices, until medium-rare (about 135F on a meat thermometer), 25 to 30 minutes. Cover the venison loosely with foil and set aside for 10 to 15 minutes before carving. Slice the venison thinly.