To get across Canada on BA/oneworld flights, you could either build in a 24+ hour stopover in the USA (some fare rules would permit this, others not, so you might need to purchase a separate one way ticket) on AA or purchase separate tickets if you want to spend less than 24 hours in some USA airport, again using AA flights. Cabotage rules say that USA carriers cannot carry passengers between two Canadian airports (or vice versa). This assumes that OP has travel documents to enter the USA.