Originally Posted by ClipperDelta
As for Oneworld and *A, I believe that the limit on international flights is only for the US carriers' members (e.g., AA has the rule that their EXPs (Emeralds)can only use the Admirals Club if they are travelling on an international (not including Mexico/Canada, etc.) itinerary; BA does not seem to have a restriction for their Gold (OW Emeralds) members using the BA lounge for domestic UK flights (though I'm not sure whether they're entitled to use the AA Admirals Club if they were travelling on a domestic US itinerary - the site is not very clear about that...)
Re: oneworld, they (BA elites) are, yes. If you're oneworld Sapphire or Emerald (mid-tier or top-tier elite) through Qantas, BA, or other non-U.S. carriers then you get lounge access day-of-flight if you're flying ("marketed and operated by") any oneworld carrier, even domestic U.S. on AA. (AA's own elites don't enjoy this benefit at AA's own lounges when flying AA domestically, probably because AA thinks it can sell a lot of club memberships without angering their elites.)(*)
The privilege extends to one guest. See
here for details.
(*) This AA-unique limitation does complicate the decision whether to accumulate miles with AA's AAdvantage program or with another oneworld partner's program. It may not be enough to tip the balance, especially for people who qualify for AAdvantage Executive Platinum (top tier), but it is a factor.