The nickname of Oscar Robertson is 'Big O'. He was an American professional basketball player who played for the Cincinnati Royals and Milwaukee Bucks. He was a 12-time All-Star and 11-time member of the All-NBA Team. He was also a part of the gold medal-winning 1960 U.S. Olympic team and a member of the Basketball Hall of Fame. He was known for his versatility and ability to play multiple positions, earning him the nickname 'Big O'.