You are correct: If the OP would be in the 777 First cabin, the Flagship Lounge would indeed be available to the OP upon arrival at ORD. (Unfortunately, only the Admirals Club has showers, though that does not preclude FL access, with its free food and beverages, of course.) And departure from BRU would allow use of the appropriate lounge (I've used both the BA and SN lounge, so it would be determined by which lounge AA currently is using there.)

As DFW has no First lounge, the Admirals Club in Terminal D would be the most likely lounge on departure (as well as the SFO AC, nothing to write home about, but with a truly top notch staff.) LHR would allow use of the AA Arrivals Lounge - though the OP should note it closes at 1400.

All this is based on an award ducat in First, and is not related to oneworld status, as I don't recall any noted.