What do you call a band whose 20-year career spans sold-out stands at both Madison Square Garden and Red Rocks Amphitheater, millions of albums sold over the course of nine full-length releases and multiple chart-busting entries? In the case of O.A.R., two simple words suffice: “The Mighty.” Since their emergence in 1996, the group has tirelessly delivered a signature brand of rock steeped in alternative scope, roots tradition and pop ambition.