Originally Posted by sc597
...I presume as I'll be coming from Asia, probably India or Pakistan, the only OneWorld way to get to Africa would be via London, which I will already have visited. Is this okay?
It is allowed, but as an exception, and only in the case of a transit without stopover on route to one of these: GHANA/NIGERIA/KENYA/UGANDA/TANZANIA.
Edited to add: otherwise you can get to JNB from HKG, or from Australia.