Prepare a grill or grill pan for medium-high heat. Grill orange halves, cut side down, until well browned, 8–10 minutes. Let cool slightly, about 10 minutes. Juice oranges, then strain and discard pulp (you should have about 1 1/2 cups juice). Pour salt onto a small plate, if using. Rub rim of an old-fashioned or rocks glass with lime wheel, then dip into salt.Stir orange juice, tequila, lime juice, and 8 cups ice in a pitcher. Divide cocktails among glasses and garnish with lime wheels.