Update: the
oneworld website now has a written policy on this.
Lounge Access by Class of Travel
Customers flying First or Business Class have access to the equivalent class of lounge regardless of their frequent flyer status. (First Class passengers may be accompanied by one guest travelling on a oneworld sold/operated flight).
Customers flying in Economy class on the domestic or intra-European segment to connect to or from a oneworld longhaul First or Business Class flight can also use the lounge applicable to the class in which they are travelling the longhaul segment.
Exception: American Airlines AAdvantage members, regardless of their tier status, may not use lounges when travelling on solely North American itineraries (defined as within or between the USA, Canada, Mexico [except Mexico City], the Bahamas, Bermuda and the Caribbean). In these cases, access rules are governed by American Airlines' own rules (ie paid lounge-access membership).