Originally Posted by Calchas

It might be a mixture of BA, AA and US flights, but they will all be issued on a single ticket by one of the carriers. (Excuse my pedantry )



Seat reservations are the responsibility of the operating carrier and can be done on the operating carrier's website for each sector individually.

If they are on BA metal, but under a different codeshare, then you may have a bit of an argument with the computer to get the seats, but at worse you can obtain seat reservations by telephone. Your silver status will apply, but there can be bugs in communicating between the carriers.