There are some exceptions to the usual oneworld rules (about being able to collect miles for one FF program, eg BA, when flying on another OW carrier, eg AA) when it comes to trans-Atlantic flights. I'm not sure what the current rules say, but it may be that you can't collect BA miles flying AA across the Atlantic and vice-versa (i.e. can't collect AA miles when flying BA over the Atlantic). Check the rules for both programs.