OneWorld Emerald Status will not get you into the BA Lounge unless you are flying on a international British Airways flight. Concourse B.
Does it have to be a BA flight rather than an AA flight? This is from the BA website:
If your oneworldŽ frequent flyer tier status is Emerald or Sapphire, or you are traveling in a First or Business Class cabin on any oneworld member airline, you can access more than 600 airport lounges throughout the world.
This means you can relax or catch up with work in comfort and style before departure, regardless of which oneworld member airline you choose to fly with.
To be eligible, you must be departing on a flight that is both marketed and operated by a oneworld member airline.
oneworld lounge access for frequent flyers by tier status
Enjoy exclusive access to premium airport lounges around the world with oneworld Emerald or Sapphire frequent flyer tier status. Members of oneworld airline frequent flyer programmes with the equivalent of oneworld Emerald or Sapphire tier status can use lounges offered by oneworld airlines when departing on any flight marketed and operated by any oneworld member airline, regardless of cabin class being flown (exceptions are noted below).
Emerald tier frequent flyers can use First Class, Business Class or frequent flyer lounges.
Sapphire tier frequent flyers are welcome in Business Class* or frequent flyer lounges. (*does not include Qantas Domestic Business Lounges)
This would seem to say that the BA lounge at IAD could be accessed during open hours by EXP (Emerald) or PLAT (Sapphire) customers on an international flight. Am I missing something? Unfortunately, my next AA international-connecting flight is before 7:00 AM from IAD.