Here is some info on getting from Jhb to Polokwane - it is unlikely that there would be a bus directly from OR Tambo airport. It is a very busy route especially at Easter time when the MASSIVE ZCC church gathering takes place, and at that time esp, a road you should avoid!! Are you going there for a specific purpose?? I would be inclined to fly rather - it is not the best route and not so good road in places.
http://www.southafricalogue.com/travel-tips/getting-between-johannesburg-and-polokwane.html almost 10 years ago
Get into the Gauteng Train
Train to Jhb Park Station or Pretoria Station
Ask Securities to the Bus station They are very friendly Most probably they will take you to bus Station and book a ticket to polokwane
Bus from Pretoia and Jnb Park Station Translux, Greyhound , City To City, Intercape and so many buses almost 3 years ago