Typically most people who are traveling to Pretoria will fly in through Johannesburg then connect to Pretoria by either flight, train or car.
Personally, the train is my best alternative it's reliable and fast. Moreover, the train has a station right within OR Tambo.
The first train usually leaves at 5.30 am while the last train leaves at 8.30 p.m.
KEY TIP; In case you arrive at the airport past around 7.00 pm don't book the train, the chances of you making it past luggage collection, immigration and checkpoints before 8. 30 p.m are very slim.
There are automatic machines where you have to print your tickets at before boarding or in case you booked online you can just show the ticket on your phone.
Tickets cost between R 20- 25 depending on seasons. However, kids under 3 years get free rides.
There's a strict no eating, drinking and chewing policy when on board the Gautrain. The ride isn't long anyway, takes about 30 minutes, you can wait till you get to Pretoria before you have something to eat.