"He did a great job for us and we are very relieved to have got away with a draw."
The captain also refused to criticise Kevin Pietersen, who was run out for 81 attempting a rash single after batting partner Jonathan Trott failed to move from the non-striker's crease.
"Mistakes happen on a cricket pitch. I would prefer to concentrate on the way those two played," said Strauss.
Man-of-the-match Graeme Swann, who took 5-110 in South Africa's first innings, admitted there was "relief more than anything" in the England camp.
"With an hour to go I didn't have my pads on and then I'm hiding out in the back room watching Onions bat it out magnificently," said the spinner, who also scored 85 in England's first innings.
Collingwood praised Onions for playing out 12 balls, including the tense final over, and revealed how he gave his Durham team-mate support in the final stages.
We played the better circket - Smith
"He keeps bragging that he's got the best bat in the dressing room ,so I kept shouting down 'Remember you've got the best bat, just keep using it'," added the all-rounder.
"I know Bunny [Onions] can hold a bat so he's certainly capable of keeping a ball out. But it's a massive pressure situation, and that's something he probably hasn't experienced before.
"I thought he was excellent. He stayed very calm, played straight and saw it through."
Onions admitted to having nerves towards the end of his innings.
"With the amount of experience that Makhaya Ntini has, all that was in the back of my mind was 'don't get out'," he said. "That's the first time I have batted with Paul, who told me to keep on fighting hard."
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