The Big O is twenty ounces of the draft beer of your choice (only $2) served in a somtimes frosted goblet at George's (commemorated in the Pat Green song George's Bar) in Waco, Texas. Shiner Bock is the best value, as it's the most expensive beer on tap. George's also has Margarit-Os, which are frozen or on-the-rocks margaritas
The drink got its name when a Baylor University student wanted to order one, but due to Baylor's Christian Mission felt embarrassed ordering a beer and asked for a big orange juice instead. The name's origins may also come from the shape of the glass.