srs, it's really unfortunate that you did not know this.

I think this has to do with the fact that AA and BA have been unable to get anti-trust immunity for their trans-Atlantic services. But I am not an expert on this issue.

I have been wondering, though, if you book an AA- or a BA-operated flight as a codeshare sold by another oneworld carrier, you might get miles credited?

AY seems to start codesharing with BA on YYZ-LHR. It was announced earlier this month on the oneworld website. I was really tempted to build this sector as an AY segment into my Explorer itinerary. I credit most of my onewworld flights to AAdvantage but I am not particularly impressed by AA's services. I prefer BA. But in the end, I took miles over my flight preference. I picked an AA flight for the transatlantic sectors.