n. — especially Montreal, Ontario, especially Sports
the nickname for Montreal's multi-purpose Olympic stadium.
Type: 4. Culturally Significant — Built for the 1976 Summer Olympics (see Canada's Games reference), this stadium was simply referred to as "Olympic Stadium" during the Games. The nickname is derived from the stadium's name and circular shape (see Image 1). Big O was not used in the province's biggest English daily newspaper, The Gazette, until 1985 (but note the 1978 quotation from the Toronto Globe and Mail). It was later humorously called "The Big Owe" because of its infamous cost overrun of $1.4 billion dollars, which took the taxpayers 30 years to pay off (see CBC reference).
See also COD-2, s.v. "Big O (2)", which is labelled "Cdn slang".
Images:

Image 1: Montreal's Olympic Stadium, the Big O. Source: Wikimedia Commons. Photo: B. Jagendorf