It's official, Oneworld is it.
Not for nothing, but everyone knew it would be OneWorld.
AAL was a founding member of that team, and there would be no way that they would, via AWE, be subservient to UAL/COA in Star Alliance. No way was that going to happen. AWE would definitely leave Star Alliance if the deal went through.
What this now does is give AAL more of a presence in Europe and possibly South America, where AWE through USA has traction. How much that will penetrate is the question.
This does not really affect them in South America - US Airways only had 2 routes to South America.
US Airways is a bigger player in the Caribbean, but AA already flies to most of those places anyway, if anything it removes a big competitor.
It does give them access to more small destinations in Europe. But I have a feeling that many of those actually depended somewhat on Star Alliance members, so it will remain to be seen how many will survive. Also, many were viable because of US's lower cost structure. However with the new AA cost structure some of these will be viable even at their pay scales.
The main thing that it gives AA is better access to the small southeastern markets through CLT, which if AA wanted to serve either had to do so through ORD or DFW (which were longer flights) or through LGA/JFK/MIA which had limited feed opportunities and were also somewhat long and in two cases, slot restricted. It also gives AA a Northeastern hub in PHL that isn't slot restricted (despite referring to LGA and JFK as hubs, AFAIK no AA domestic destinations that were served from JFK or LGA were only served by one of those airports - they all had ORD, DFW, or MIA flights. AA's operation at LGA also sucks for connecting traffic because of their split operation across two concourses that requires reclearing security.
It also gives AA a big presence in Washington DC, which they had given up as part of the cornerstone strategy.
It also helps solidify their position in Mexico - both were far behind UA among U.S. carriers to Mexico. The combination of the nearby hubs in both PHX (covering western Mexico) and DFW (covering eastern/central mexico) to reach Mexico and the big O&D Markets in ORD and JFK should make the new AA a lot stronger in Mexico.
Keep in mind that in many ways US is acquiring AA though. From their perspective, it gains them access to the New York market, a huge hub in DFW, and those huge Joint ventures with JAL and British Airways.