This time BA as the RTW about six months ago I used AA on LHR-MIA (and was suitably underwhelmed), plus I find that the AA RTW team doesn't seem to enforce a "you must use AA across an ocean", but rather "you must have at least one AA flight" on First class at least.

I don't think there is any other option on oneworld except AA and BA to cross the Atlantic now - Qatar who does lots of US flights only offers business to the US and JAL finishes at LHR and JFK respectively (and won't let you book anyway...)?

(My cross ocean legs are LHR-JFK-LAX-HKG on BA-AA TransContinental-Cathay - intention is to maximise Tier Points with very limited time hence no random trips to ANC or SJU)