I'm not really familiar with OneWorld, but looking at AA's website, it looks like they grant miles based on the marketing carrier's flight number. If AA sells you the flight with the AA number (regardless of whether it's a BA-operated or AA-operated flight), you'll earn miles per AA's chart. If you book the flight with the BA number, you'll earn according to the BA chart.

I'm not sure what it would look like if you were in BAEC, Ethiad, or CX. That's going to take some researching on your part.

Generally speaking, though, the big three alliances have aligned their policies. I know for Skyteam, miles earned is based on the marketing carrier. For Star Alliance, miles earned is based on the operating carrier, regardless of the marketing carrier. I would guess (but I can't guarantee) that OneWorld grants miles based on marketing carrier, based on AA's system. And do be aware that outside of the alliances, codeshare and frequent flier partners can have all sorts of earning charts--it won't necessarily be the same for each partner.

Hope this gives you a start.