The judge, sitting at Isleworth Crown Court, west London, noted that Oldfield smiled as she spoke.
But as she sentenced him to six months behind bars, he looked stunned and slowly shook his head from side to side.
Oldfield was convicted of causing a public nuisance after throwing himself into the River Thames on April 7 this year, bringing a sudden halt to the annual Oxford and Cambridge boat race.
He told the court during his trial that the race was a symbol of elitism in government.
But Judge Molyneux said he had shown no regard for the thousands who had gathered on the banks of the Thames to watch the race, the many others watching at home on television or the sacrifices of the rowers whom had trained for many months.
She noted that the first thought of all those who came to his help was to protect his safety and that the event was a "free spectacle open to all".