His teammates rushed to help him as he fell backwards.
Tainui was standing in the softball dugout watching as umpire Tamati Montgomery carried out CPR on her son while they waited for St John ambulance to arrive.
Ollie, a student at Wesley College, was recovering at Middlemore Hospital and tests were being carried out on him to determine the cause.
But Tainui said having Montgomery who is trained in CPR on site and having an ambulance nearby saved her son's life.
"We are totally indebted to him," his mum told Stuff. She also thanked his teammates and the paramedics for helping Ollie on Saturday.
Ollie was selected last year for the ISA New Zealand Youth 16 Boys Team tour of Denmark and the Czech Republic.