They have a sign outside BA lounge stating it is primarily for BA66 but they will try and accommodate other OneWorld space admitting. I am on AA and was there quite early and no problems getting in, I asked the lady and she said they keep tab on how busy the BA66 flight is (today it is full, oversold in Prem Eco even) and then count in any OneWorld - she would not share how many "others" they would let in. Both BA and AA flights leave around 1830-1845.