Unfortunately, your case is a well-known exception to the general oneworld system.
AAdvantage members do not earn mileage for BA flights between the Americas (North *and* South) and London.
It's unfortunate that you did not know/were not informed of this rule.
At this point, there is little that you can do. For next time, try crediting the trans-atlantic portion to BA USA or another oneworld carrier.

[This message has been edited by megamiles (edited 11-23-2000).]