Unfortunately, there is no Oneworld flight South Africa to South America - only SAA and Malaysian cover that route, with Varig and Aerolineas Argentinas code-sharing. I'm pretty certain Cathay do not code-share onthe JNB-EZE SAA flight although they do on HKG-JNB as well as having there own flights. Even if they did code-share though, it wouldn't be allowed on the Oneworld RTW fare which excludes code-shares not operated by non-Oneworld carriers.

Back to the original question though,there are other options to surfacing from NBO to JNB.As you are allowed 4 segments within Africa you can do a side-trip NBO to the Seychelles and back on BA then fly the next day from NBO to Mauritius from where one-way flights to Capetown, Durban or JNB on SAA are only $US150 or so. Alternatively, BA (Comair franchise which is included in Oneworld) have flights from Lusaka, Vic Falls and Harare to JNB so you could surface to any of those and pick up a flight. They used to fly to LLW in Malawi but I think they may have pulled out of that route.