O'Leary won the John Mackey Award as the nation's top tight end in 2014 with Florida State. He finished his career with the Seminoles as the school's all-time leader among tight ends in receptions (114), yards (1,591) and touchdowns (18).
Lewis was a three-time All-Southland conference selection at Central Arkansas. The 6-foot-4, 212-pound wide receiver finished his college career with 197 receptions for 2,618 yards and 24 touchdowns.
O'Leary and Lewis are the first of Buffalo's six draft picks to sign with the team.