Instead it was the Springboks who stole the show and with it a series victory as Morne Steyn's last-minute penalty gave them their second successive triumph over O'Connell and his team.
"Right now that feels like the hardest defeat I've ever had to take," said O'Connell.
"It was very disappointing to concede a late try that we should never have conceded and then to concede the penalty at the end is heartbreaking."
O'Connell and co were in complete control throughout the majority of the first-half but somehow the Boks forced their way back into contention as the second period progressed.
Munster captain O'Connell admitted he could not fault the effort of his team-mates but pointed to a failure to play the way they wanted for a full 80 minutes as the reason behind a heartbreaking 28-25 reverse in Pretoria.
"We had a lot of injuries and that told," added O'Connell.
"We lost Brian O'Driscoll who was the lynchpin of the team. That was tough but we just didn't play enough in the second half.
"We were all over them when we did play in the first half. That was the way I knew we could play on this tour. But we needed to play like that for 80 minutes and we just didn't get going in the second half.
"I thought we had done enough to at least get a draw and keep the series alive but that has gone now. But the commitment from the boys was incredible. The boys were so proud to play for the Lions and wanted to win for the Lions."