The term 'Yankee Opponent' refers to two teams that have a long-standing rivalry in Major League Baseball. The first team is the Baltimore Orioles, which has six letters and is often referred to as the O's. The second team is the Rebel, which has three letters and is more commonly known as the Atlanta Braves. Both teams have been competing against each other since the late 19th century and have had many memorable games over the years.