The court heard that Oldfield, an Australian who moved to the UK in 2001, decided to make the protest after learning of Government plans to "sell off" the NHS and "snoop" on electronic communications, and after hearing encouragement given to "dob in" people planning protests during the Olympics.
Oldfield, of Myrdle Street, east London, worked and volunteered for a decade in jobs and projects aimed at increasing better prospects for people in impoverished areas.
He stopped the annual contest for around half an hour, the first time in the history of the 158-year event that it had been disrupted by a bather.
Prosecutor Louis Mably told jurors the race between Oxford and Cambridge was spoiled for hundreds of thousands of spectators watching from the banks of the river or live on BBC TV, not to mention the two university rowing teams.