Ojo will arrive at England's training base in Centurion on Thursday, providing back-three cover, alongside fellow newcomer Nick Abendanon.
Bath full-back Abendanon flew from London last night as England continue to be affected by injuries and a stomach virus that has swept through the camp.
England head coach Brian Ashton will not announce his starting line-up for Saturday's second Test at Loftus Versfeld until Friday, such has been the extent of disruption.
Saracens back Dan Scarbrough and Bath prop Matt Stevens joined the squad last week, and both players look set to start this weekend.
Ashton said: "Because of the shortage of backs I highlighted yesterday, I decided to call up Topsy.
"He was on stand-by for the England Saxons and will provide emergency back three cover for this weekend.
"I didn't want to further disrupt the Saxons starting XV (against Barclays Churchill Cup final opponents New Zealand Maori) for Saturday by taking another one of their players."
Ashton on Thursday ruled out Andy Farrell, Mike Brown, James Simpson-Daniel and Andy Hazell from his selection plans, while skipper Jason Robinson is battling to overcome a knee injury.