If you are a oneworld sapphire member – think British Airways Silver – you get lounge access domestically within the U.S. even though American Airlines frequent flyers don’t.
In fact oneworld sapphires, which is the mid-tier elite level, get American’s Flagship lounge (international business class lounge) access on U.S. domestic itineraries. American gets a carve-out in oneworld rules so they do not have to offer this to their own members.