I recently had to spend the day at OR Tambo having arrived on the 55.
I booked a day room at the City Lodge hotel which is walking distance from the terminal for about £35. The room was clean, had a good shower and was perfect for a good sleep after the overnight flight.
When I woke up I headed off to Quills restaurant at the Intercontinental Hotel which is next door to the City Lodge for lunch.
The BA lounge opens at around 3.30pm / 4pm so you can then head back there.