Originally Posted by
TabTraveller
"Any J passenger traveling on any oneworld airline can go to any lounge operated by any oneworld airline in the same terminal"
Doesn't have to be in the same terminal at all. For instance - TBIT lounges when travelling from T4 LAX. Generally as long as you can get their airside then you're fine to access.
In LHR, it does have to be the same terminal (except T5A/T5B, which are considered the same terminal). The exception is the American Airlines - Admirals Club in Terminal 3, which is in place for their members but seems more broadly applied.
That said, I did successfully access the MH Golden Lounge on a BA flight, and got the impression from the highly confused lounge person that I was the first person that year to have done so.