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 7 years ago