Minutes later, wrapped in a blanket and grinning broadly, Mr Oldfield was led away by police, and was being held last night on suspicion of committing a public order offence.
It was without doubt the most bizarre moment in the 158 years of the Boat Race, which had previously seen six sinkings and one restart.
John Garrett, the umpire who halted the race, said: "Sir Matthew basically said, 'There's something in the water, there's something in the water.' He thought it was some debris and then we realised that it was actually a swimmer.
"We weren't sure what was going to happen, whether he was going to get out of the way in time and then it was quite clear he was just waiting for the boats to come across him so I had to stop the race and restart."
On the Chiswick side of the river was Annie Osborne, a spectator. Spotting what she thought was an animal in the water, she grabbed her father's binoculars to get a better view. "When we first spotted him, we thought he was a dog, until we looked through my father's binoculars," she said.
"He was at the river bank and after a while it became clear he was waiting for the boats to come around, before swimming out to them.
"When he was brought onshore at Chiswick Pier, he didn't say anything, but he looked very pleased with himself."
Sgt Chris Tranter, of the Metropolitan Police, said the rowers had nearly decapitated the swimmer. "They almost took his head off," he said.
Sir Matthew said: "It might have been a very serious injury. Fortunately, we spotted him and stopped the race. It's not ideal but what could we do? We could not possibly have carried on."
In his wake was confusion and chaos. Oxford were a quarter of a length in the lead when Mr Oldfield brought the four-and-a-quarter mile race to a standstill.
Shortly after the race was restarted, just before 3pm, Oxford suffered a broken oar, apparently caused when they clashed with the Cambridge boat.
Handicapped by their broken blade Oxford, who had been favourites, went on to limp in second.
That wasn't the end of the drama. At the end of the race, Oxford's bow man Dr Alexander Woods, 27, collapsed and had to be lifted from the boat by medical staff.
He was taken to Charing Cross Hospital, where he was last night said to be in a stable condition.
Meanwhile, Mr Oldfield's identity was becoming public, as well as his reasons for the stunt, which he claimed was a deliberate act of civil disobedience.
Shortly before taking to the water Mr Oldfield, who was apparently privately educated before studying at the London School of Economics and is a Fellow of the Royal Society of Arts, posted a manifesto online.
Entitled Elitism Leads to Tyranny, he declared: "This is a protest, an act of civil disobedience, a methodology of refusing and resistance.
"This act has employed guerrilla tactics. I am swimming into the boats in the hope I can stop them from completing the race."
In his manifesto, Mr Oldfield said he was targeting the boat race because of its elitist nature - saying it went past Fulham Palace, a former royal residence, St Paul's, the leading public school, and the London home of Nick Clegg, the Deputy Prime Minister, who he attacked for being educated at public school.
He even likened it to the actions of Emily Davison, the suffragette who threw herself under the King's horse at the Epsom Derby on June 4, 1913 and died of her injuries four days later.
Mr Oldfield is familiar with this stretch of the Thames, having once worked as a coordinator for the Thames Strategy project, set up to regenerate the landscape around the river from Kew to Chelsea.
He is also a member of a group of activists called This Is Not A Gateway, which he set up with his girlfriend, Deepa Naik, a fellow artist, to protest at the way modern cities are run.
Mr Oldfield, who lives in a 1930s apartment block in Whitechapel, east London, also objects to "shiny buildings" and wants to "remove every fence from around every park".
His blog urges people to stage similar stunts in a bid to disrupt the London Olympics and suggests, somewhat bizarrely, that taxi drivers take their passengers the slowest and most expensive route possible in protest at their "elite status".
Last night, he was being widely mocked on the internet, the medium he used to post his manifesto - although his actions are being more seriously studied by the organisers of the Olympics.
There are particular concerns over the security of "open" events such as the cycle road races and the marathons - which will take place outside of the Olympic Park in Stratford also in east London - and could be targeted by pranksters as well as serious protesters and even terrorists.
A spokesman for the London Olympics said: "There has been an increase in protests, such as those by the Occupy movement, which has already led police and security agencies to assess how to counter this kind of incident."
Oxford, the losing team, did not see any funny side in the bizarre protest.
Karl Hudspith, president of the Oxford University Boat Club, wrote on Twitter: 'To Trenton Oldfiled [sic]; my team went through seven months of hell, this was the culmination of our careers and you took it from us."
Mr Oldfield left Chiswick Police station shortly before midnight.
No longer in a wetsuit, he appeared in an expensive-looking black peacoat, crisp white shirt and navy trousers. He seemed, however, to have forgotten his socks.
His only comment was "It's a protest. Read the blog."
He was driven away by a blonde woman.
A Scotland Yard spokesman said Mr Oldfield had been charged with a Section 5 offence under the Public Order Act and bailed to appear at Feltham Magistrates' Court on April 23.
• Additional reporting by Clare Hill
