Oldfield, 36, who moved to east London from Australia, interrupted the April 7 race by swimming into the path of the boats in what he said was a protest against government cuts.
He has also been told to pay £750 costs.
Judge Anne Molyneux noted that Oldfield had acted dangerously and hadn’t shown what he was protesting against.
A statement from assistant umpire for the race, eventually won by Cambridge, and four-time Olympic gold medallist Sir Matthew Pinsent said Oldfield risked his life with the stunt.
‘He could have been killed if he had been struck by an oar or the rigging, which is metal,’ prosecutor Louis Mably read to the court.
The rowing crews were followed by 25 motorised boats carrying officials and spectators.
Oldfield said he was demonstrating government public spending cuts, claiming they were ‘worse than in Dickens’s time’.
The judge said he was sabotaging the 158th edition of the race on the Thames because he considered it elitist.
Oldfield said: ‘It’s a symbol of a lot of issues in Britain around class. 70 per cent of government pushing through very significant cuts are Oxford or Cambridge graduates.
‘It was a symbolic gesture to these kind of issues.’
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